I'm having the exact same issue on our kid's accounts with Parental Controls enabled. The odd thing, it doesn't effect all of them. I think it's happening with any
login screen....Facebook, MobileMe and even the support site here generated the message. I had to switch to the admin account to even login to submit this.
What I've done so far is to double/triple checked the Parental Controls settings, added the offending sites to the list of "allowed" sites. I've also deleted cookies, reset safari, etc. I even tried FireFox (same issue) so it's not browser-specific but appears to be a bug with Parental Controls.
It appears to be something to do with OS 10.6.4 and Parental Controls. Everything has worked peachy until the last few days. It seems to roughly conicide with the install of 10.6.4 (installed the day it was released) but my daughter tells me FaceBook just stopped working today (MobileMe login has not worked for some time).