There's a weird workaround, though. To disable the attribute, use the uppercase letter (instead of lowercase, as it should be). To enable the attribute, write the lowercase letter of the attribute followed immediately by the uppercase letter of the same attribute.
Here's an example for Invisibility:
Enable: SetFile -a vV some_file
Disable: SetFile -a V some_file
[robg adds: Hopefully they'll fix this glitch in the next XCode update.]

