User!UserID not working in conjunction with Available Values

I have been banging my head against the wall over this now for a day or two.

I also found a thread with my same problem but no conclusion (http://social.msdn.microsoft.com/forums/en-us/sqlreportingservices/thread/60DD8B17-48EF-473B-9C05-DD5BBEA8B84B)

My problem is this. I am able to use the User!UserID as a default value, with NO available values set, and it detects the current user and runs the report without any trouble.

However, we want to be able to provide the list of available people for users to choose from, but by default set it to themselves. Once I bring Available Values back into the picture, it no longer detects the current user.

For the 'Default Value' in my parameter I have:

=Cstr(LCase(Trim(User!UserID)))

For the UserID (calculated field) populating my available values I have:

=CStr(LCase(Trim(Fields!LoginUser.Value)))  

Any idea what the heck I might be missing here?