I recently installed WordPress MU (2.7) and have it up and running. I posted this question there, but was told it's an issue in the non-MU version too (so please excuse the re-post)
When I use the TinyMCE spellcheck on a post it responds with "No Misspellings Found" even when there are misspellings.
I have not edited any of the files that came with the install, so I believe it should be using Google to check the spelling.
I'm trying to get this to work for English, although I don't think it works for other languages in my install either.
Some notes: Installed on Linux; SSL required; Viewed with current versions of Firefox and Safari on a Mac