According to Audrey Zolghadr (French to English translation courtesy of Google's translate services), the WinMo product manager at Microsoft France, the Windows Mobile platform currently has over 20,000 apps available for it. The Marketplace for Mobile site will work with the software on the device to ensure that the user only sees software that will work with their device. For example, not all software works with all screen resolutions or works with both touch and non-touch screens.
I do wonder why only 600 apps will be available when 20,000 exist. Are developers reluctant to pay the $99 fee or is Microsoft really focusing on getting the best apps in the store for launch? The one thing I'd really like to see change hasn't and that is the marketplace will only be available for Windows Mobile 6.5 devices. No existing phones will be able to use it unless it is on the short list of devices that can be upgraded, like the Touch Pro2.
The store will launch later this year concurrently with Windows Mobile 6.5. I've not seen any dates yet. WinMo 6.5 is done, it is just a matter of the manufacturers and carriers putting their finishing touches on it and finalizing testing.