Follow along with the video below to see how to install our site as a web app on your home screen.
Note: This feature may not be available in some browsers.

Well, I finally found a cure. After trying all the various workarounds on the web (including the clear cache and data on market), I went through the following:
Went into airplane mode (just to stop any sync issues)
Cleared all possible cache/data on the following:
Market
Market updater
Download manager
Google Mail + Google * (i.e. any other google apps)
HTC Checkin Service
Talk
Then I turned off phone. Turned back on. Then took out of airplane mode. Then market started working. Must be an issue with one of the other google apps which uses my creds.
BTW, I had previously just cleared cache/data for the market app (and force closed it) and market updater and download manager and chat. Those didn't work by themselves. When I cleared out anything which pulls on the google account it all started working again.