The big problem with spell checkers is the dictionary list. I have one for a text editor I use that is by no means complete, and it's 1.3M in size. With client side Javascript, the client has to download the whole file before using it. With T1 or equivalent connections, this would take a few seconds. With a standard 56K connection, this would take over 3 minutes at the top 56K speed. With worse connections, it could take 7 or 8 minutes to download. And this is JUST a fairly decent dictionary.