I remember a similiar argument about wheather the use of digital calculators in schools would diminish the ability of doing math in your head or on paper. The answer was more or less: Of course it does, but it doesn't matter. The chances of anyone ever finding himself in a situation where he has to do complicated math and not having a calculator in reach are converging towards zero.
Similiarly, I think it can be said: Yes, spell checkers reduce the ability of children to correct their own writing, but the chances that they'll ever have to write anything (other than *maybe* a love letter, but I hear even those you may do electronically today, a thing that was still a no-go in my youth) without having a spellchecker around are virtually non-existant.
Which reminds me, it might be a good thing if I would finally install one. I never used one in my entire life, and the results clearly show. Then again, I'm regularly writing in 3 different languages, and it's kind of a pain to always switch the thing...