My son, 11, is also a poor speller. However, he is taking Spanish this year and has homework on the computer, where he has to type the answer in Spanish correctly, including the accents. He's learning Spanish spelling fast because of the repetition and the instant feedback, and because the computer keeps giving you the same words until you spell them correctly several times. The first few lessons took over an hour each. Now he usualy finishes in 20 minutes or so.
Makes me think we should have had some sort of English spelling program like this: maybe the computer could "say" the word and the child could type it. Together with learning the "rules" of spelling (which are so funky in English!), this might have really helped.
We've settled into this with English spelling: as long as you can recognize when words don't look right, and will look them up in a dictionary for things you are going to hand in, I don't really care too much. He uses a small "word finder" dictionary in class--it just has the words, not definitions, so it is more compact and faster to look up words. Eventually he'll be typing most things and will be able to use spell check.