I was going to write a post about Spolsky's latest (Don't hide or disable menu items). I'm unashamedly a big Joel Spolsky fan (I have the books, I dream of running a company like his, blah blah blah). However I didn't agree with his post (Spolsky says you should always leave menu items enabled and use error messages if they are clicked at an inappropriate time).
Some other guy wrote a good retort to Spolsky:http://www.red-sweater.com/blog/515/disabled-menus-are-usable (hence my need not to write a long post).
Personally I prefer to disable and also to change to tooltip. It should also note I'm not a fan of enhanced or 'super' tooltips in either thin or fat clients. Just plain old tooltips will do fine.