Instead of a spell checker specifically for WordPress, I use the SpellBound Extension for FireFox. For me, this is a better solution as it can be used to spell check ANY text input window (including this one that I am using to enter this post). That way I only have one custom dictionary that I have to keep up with no matter where I am entering text. There is a new beta version that I have been using for a couple of weeks with no problem that even includes on-the-fly inline spell checking much like Microsoft Word. It is available here.
That said I too wish I could get a spell checker to work in one of my multi-user blogs. Most other users would just rather have a spell checker built in instead of loading yet "ANOTHER piece of software." This blog is on a windows IIS machine, and I have yet to find a spell checker for that. So try SpellBound out, you will love it!