Ok I noticed this glitch as well. So What I did was I made another limited account on the Lion Machine. went to Sharing and then I then selected the shared folder(s) that I wanted and put that account that account only any other account has no access. I selected that account as the SMB, and typed in the password. Then I restarted the Lion Mac. Went back to Sys preferences and was able to deselect the Admin account.
On the windows 7 machine It then shouldn't let you just go to Network and select you Lion Mac Mini. and it should ask you for the username and password.
Select option B "Use different credentials."
then log on using that account and password to the Mac share(s).
Let me know if that helps at all.