I have the same problem, but not with accents ´ ` but with characters like ô ç â , when I use it to write in french and I take the write assistant option. I had Nokia from years ago (N80, N82, even asha 303 recently) and the're an option to change which type of characters to use, if "complete" or "reduced" or some thing like that, and choosing "reduced" fixed it the problem, but in N900 I didn't find this option (if exist)