There's no better proof that you don't need a current SDK to publish apps for Windows Mobile 6.5 than the Marketplace, but that's not to say there aren't specific bits new to 6.5 that devs might like to use -- problem is, there's never been a final SDK that Redmond's made available. That changed -- ever so briefly -- on Friday as the company posted both Standard and Professional Edition SDKs in a variety of languages, but by today, they've been pulled. No official explanation has been given, but there are a couple solid theories: one, the SDK outed 6.5.3's full details (it's still not clear whether Microsoft has "officially" unveiled this update or if it'll be throwing a bigger launch party, perhaps at MWC), and two, there are apparently some incompatibilities with newer versions of Visual Studio, which is required for the SDK to function. Stay tuned, but in the meantime, just keep on developing the way you've always been doing it and you should be fine.
Update: So the::unwiredheard back from Microsoft on the issue, and apparently, the SDK was "prematurely" posted in beta form and pulled once the company realized the mistake. Of course, this explanation could realistically be a cover for virtually anything, so we'll take it with a grain of salt.