So, how is this in comparison with other offerings? Apple's App Store is leading the front, as it's been around the longest, with over 50,000 applications (although apparently some of those have been deleted since). Nokia's Ovi Store has "over 20,000 items", but of course most of that is not applications, but wallpapers and the like. Additionally, we have Palm's App Catalog which has barely any at all. What you have to remember, though, is that (according to PhoneArena) Windows Mobile has around 20,000 applications currently available anyway; that '600' figure only counts the ones that will be on the store, of course.
Microsoft has put together a video about developers and how they want them to start submitting applications right now (we've included it below). There's also the official website for doing so right here. Feel free to leave thoughts below, as per usual.