FWIW I'm seeing similar issues with 6.30
Selecting multiple controls on a page and using Tools | Duplicate Control causes an error (duplicating the label name so the form won't reload in the designer)
I've also noticed seemingly at random everything is fine but then I can't can back into the designer with various different error messages blocking it (and most of the object/control tree disappearing as I guess it fails to parse the form)
Not sure if it could be Vista SP1 related?
If I then open the source in notepad and check for the following things I can usually fix it
- raddmenu instead of admenu
- baddform instead of addform
- multiple "end sub" at the end of the designer section (remove all but one)
- search for (and rename) duplicate label, textbox etc
Would be nice if it didn't do it in the first place though