Ah, you caught me. I'm way out of practice ^^;;
"コンピューター"
Sure enough, it's my spelling which was at fault.
Some documentation on the key commands available would be handy - I was able to figure out a bit about the space bar and arrow keys but I'm sure I'm missing some features. I'm on my Ubuntu/Firefox box at work currently so that's the environment I'm working in. I don't currently have an IME set up here. I did have difficulty trying to type romaji, such as "typd.in" in the midst of the kana. A shortcut key to transpose directly to katakana would be handy. Trailing 'n's didn't convert directly to hiragana while typing in most cases - I had to follow with another character to get it to take, which was kind of jarring.
The bookmarklet is very clever. I'm going to post it to a message board I frequent which has lots of aspiring Japanese students, if that's ok. I'm sure you can get some good feedback from them.
One thing that would be a killer-app to someone like me whose writing is atrocious would be a feature to display definitions of kana in both english and japanese when displaying possible readings. Sort of like an on-the-fly rikaichan.
Anyway, very cool app. Good luck!