if whatever you're building on the primary domain is successful, you'll end up purchasing the variations down the road anyway and usually at a huge premium because someone else will have bought them up to hitch a ride on the gravy train. Might as well buy them early and save money.
I really disagreee with this. If you become massively sucessful on a myspace/google/facebook level then perhaps. But has pg bought yombinator.com? Where does dig.com take you? What about gamil.com? twiter.com? micorsoft.com?
Forget about the misspellings, you're not going to lose a lot of traffic to them, people will soon realise they're in the wrong place. Save your money for better things like advertising and servers.
del.icio.us was bought by yahoo before it became delicious.com. how many people typed the wrong domain in? Quite a few probably. Not enough to stop it being a huge success, not enough to prevent it being acquired by yahoo. I think it's a non-issue.
