I've been a TextMate user since... well as long as I can remember (I was so glad to be done with all those BBEdit trials– seemed liked every time a new MacWorld magazine was out it would come with the latest and greatest... BBEdit). But, over the last year or two, this is the one problem that has caused me to revert multiple times to other editors. I was hoping TM2 would improve on this a bit but so far nothing's happened.
No, seriously. Typing Japanese characters in TextMate is a worse experience than, say, typing them into a terminal window, or typing them into an aged Carbon application that was obviously not designed for Japanese input.