That URL looks appropriate (assuming that you aren't literally putting the username in quotes) - it matches what I'm using.
It doesn't quite match your first post, however - there it appears you had this URL:
http://idisk.me.com/USERNAME/Documents/Brian/
I'm guessing that you have simply removed the trailing "Brian/" since your first post, and that this doesn't actually indicate a weird bug.
My suggestion would be to mount your iDisk in Finder, move aside the OmniFocus document there (assuming one is there), and try again, just in case there is some kind of issue with the document that is causing Apple's WebDAV some problem.