In an exploding market where an estimated $6.2 billion will be spent on 4.5 billion mobile apps this year alone, consumers find themselves essentially deluged with a large selection of smartphones powered by an increasing number of mobile operating systems from Apple (AAPL), Google (GOOG), RIM (RIMM), HP’s Palm (HPQ) and now Microsoft (MSFT).

With Windows Phone 7-loaded devices just launched to the public, the software company that helped define the PC is hoping it can wipe its slate clean and start anew in mobile. Critics so far agree that the interface, an unorthodox unified tile-based menu system, is fluid and easy enough to navigate, but the bigger question remains: will mobile application developers back a new OS in a space dominated by Apple and Google?

For answers, Fortune looked to the developers featured in our earlier story, The New Kings of Apps, whose innovations are a big part of the reason consumers demand high caliber apps on their smartphones. So far, the developers we spoke with are keeping an eye on the new platform, and sound positive about Microsoft’s work on the OS — but that’s not translating into allocating resources to it — for now.

Pulse News Reader from Alphonso Labs
Co-founder Akshay Kothari and Ankit Gupta

What: Since the slick RSS reader launched earlier this year, the product of Stanford grad students Akshay Kothari and Ankit Gupta continues to top the iTunes Top 10 for paid news apps. The staff has grown from two to six, and an Android app launched last July.

WP7 pros: According to Kothari and Gupta, Microsoft has really achieved a beautiful interface and design with their new OS, and given that it’s backed by one of the largest tech companies in the industry, the potential for a viable app ecosystem is unequivocally there.

WP7 cons: Yet another mobile OS to support. “To be frank, we’re really constrained by the law of demand as app developers,” says Kothari. “Eventually we want to be cross-platform, but at the same time, I think it’ll be very hard for our company to have say, five different apps that you want to support completely.”

The verdict: Microsoft has a stunning product, it may be a few years too late. Kothari and Gupta don’t rule out the idea of one day developing a proper WP7 app, but their priority remains iOS and Android. Says Kothari: “As soon as we get reinforcements, I think we’d be interested in prototyping it and at least seeing if we can do something interesting there.”

Square
Co-founder Jack Dorsey

What: The startup, which transforms mobile devices into credit card readers with a free square-shaped dongle, is exiting its experimental pilot phase. It’s also still learning from some of the unexpected challenges caused by reinventing the credit industry, such as minimizing risk and fraud potential. On the flip side, analytics software since day one has helped Dorsey and crew better understand merchant and consumer behavior.

WP7 pros: Dorsey admires the interface’s focus on bigger, larger more graphical touch targets. If they can properly utilize them and focus on making onscreen information as clear as possible, he believes Microsoft will not only have something people can understand right away, but a product they’ll want to come back to, something that has traditionally not been the case for Windows devices.

WP7 cons: “I think they have a lot of catching up to do,” he says. “As Apple has shown, really focusing on the user experience and an app store has worked out very well, so that’s obviously a model that other companies want to replicate. I imagine that Microsoft has to get into the store aspect and make that fluid and really dynamic. That’s really important to do right away.”

The verdict: Wait and see. If there’s massive demand for Square on WP7 devices, then Square will invest.

Layar (formerly SPRXmobile)
Co-founder Maarten Lens-FitzGerald

What: The Layar app allows Android and iPhone owners who download the free app to also download “content layers,” or extra interfaces from different companies to help users find useful — or sometimes just plain entertaining — points of reference like say, the nearest bank ATMs, shops, restaurants and user reviews, all seamlessly overlaid on real-time street views thanks to their phone’s GPS and camera. The company says over 100 outside companies are now developing content layers, including Wikipedia, Twitter, and Brightkite.

WP7 pros: “We’re very happy that Microsoft is getting back up to speed,” says Lens-FitzGerald. “You and I know how they were like when they were in the 90s, but with Mobile 7, it looks like they’re really gearing up to [do] something new.”

WP7 cons: Because Microsoft currently does not allow access to the APIs controlling cameras on WP7 devices, the company cannot develop a version of their app for the OS.

The verdict: Pass — at least until Microsoft wizens up, or as Lens-FitzGerald puts simply: “Give us access to the camera.”

Fring
Vice President of Marketing Jake Levant

What: The free cross-platform mobile call and video chat app from founders Avi Shechter, Boaz Zilberman and Alex Nerst still puts Apple’s FaceTime to shame. According to Levant, Fring picks up at least half a million new users a month. And this month’s update to the Android app, which allows free voice calls and lower-cost calls to international land lines, should further help their cause.

WP7 pros: Some may argue that the Microsoft legacy could hurt the OS, but Levant believes the company’s reputation will bring new smart phone manufacturers to the table.

WP7 cons: Developing an ecosystem, as Microsoft must do with apps and phones, will be tough. “Microsoft has a lot of willpower, and it has ways, but they’re not the only company that’s vying for the attention of 250,000 app developers,” he says. Also, the lack of a front-facing camera on any of the announced WP7 devices is a glaring omission. Where’s the fun in video chatting if you can’t see the other person and vice versa?

The verdict: Levant says the company is taking a “keep learning and keep abreast” approach with regards to WP7, a more positive  way of saying (again) “wait and see.”