Yes, built in is always the best.
It would be nice to have that option.
The thing is that it can be a huger resource hog. Here is an idea, though it should be more like a suggestion directly to InfoPop:
Infopop host a spellchecking software that blends into our boards (Im sure the other UBB boards would also like that). They could even filter the IPs to allow only registered software.