I got this too, and it seems to be the only site on the web that Google has indexed on this issue. I declined to enter my password, and the next box I got was one saying "transition is trying to modify your user account." I canceled this one too.
I then did a "sudo ps -ax | grep transition" and found the following file: /usr/libexec/transition. I didn't see any information on this online, but running it gave me this line:
2012-07-27 13:51:32.895 transition[3583:707] INFO: Not signed into MobileMe, nothing to do.
This was on my Mac Pro running Mountian Lion, on which I didn't get the warning. So, I got the warning on my MacBook Pro, but the file was also located on my Mac Pro.
Given that it's on both of my computers but I only got the warning on one, I think it's an innocuous message box. Now I wish I'd entered my password and I wonder what's not going to work as a result. Perhaps it's a permissions issue, although repairing permissions didn't highlight any obviously related problem.
