Sean, this is a new one to me. You might try deselecting the "Save Connection Details" link in the connection dialog. That bypasses the registry settings.
I wonder if there's an extra space in the hostname or username. On your Win7 system have you tried re-typing all the fields in the connection dialog?
If you get into this situation again (or your Win7 system is still not
working), shoot me an email to joe-at-f5-dot-com with the contents of
the "\HKCU\Software\F5 Networks\iRuler\Accounts" registry hive so I can
try to see if there's some corruption going on in there.
If you want, I could setup a debug build for you that did some internal diagnostics during the connection process to figure this out.
-Joe