Not 100% sure yet, but I believe purchases would still only be valid on one device at a time. If we can find a way to reliably read SD card serial numbers on Windows, though, we might be able to pull the same "Card ID" trick from Palm/WM on desktops as well.
And yes, I agree completely about the overall quality of desktop apps - in fact I think both Apple and Google have been ignoring this at their peril by failing to do more to encourage higher-priced / higher-quality apps for their mobile platforms; they've left open an opportunity for Microsoft + Intel (not in 2010, but maybe in 2011/2012) to introduce an ultra-low-power Atom smartphone running Windows 7. Applications would still have to be tweaked to offer good small touchscreen interfaces, but doing that would be a whole lot easier than rewriting them for iPhone or Android or Windows Phone 7 (heck, some Windows applications are so customizable that the user could practically do the customization by him/herself), and their capabilities would dwarf anything in mobile apps - why settle for Photoshop Online when you can run the actual Photoshop? Why play Civilization Revolution when you can play full-on Civ IV? (Steam on smartphones is a prospect that should excite any electronic gaming fan) Microsoft Office versus iWork / QuickOffice / DocsToGo / etc isn't even a fair fight...
And with tablets the Windows + Atom combination is pretty much a slam dunk even now - no major UI changes needed in a lot of cases. Windows 7 may not quite have the slickness / speed of an iPhone- or Chrome-OS-based tablet, but applications-wise there's just no comparison. Though for more casual users the polish / ease-of-use of an iPad would still make it a better bet.