You may have gotten the feeling from my latest flurry of cranky and short IPFS related posts that I've been struggling with browser forms a bit lately. You would be correct.

I had a 2010 form with code that I published as sandboxed to my SharePoint 2010 install. I was using the userName() function in a loading rule (to set a field to the user's name, of course). My form threw an error on open - you can find a similar issue outlined here. Apparently the necessary SPClaimProviderManager was not available for sandboxed code.

In our environment, the December 2011 cumulative updates fixed the issue.