i have a .net mistype that does ok in sedo, admittedly its a 5 letter domain, using a very popular (adult) term..Certainly a keeper, but i think this would be rare, like most people have stated, 90% would be .coms. In terms of securing mistypes, its best to keep up to date on the news and watch sites grow alongside topical news. Mistypes from very well known websites are basically all gone, or will cost you a pretty penny to buy.
Read up on the latest trends...look at the sites bringing news and views on those trends/topics, and take a stab on the ones you think will be popular..keeping in mind length of domain, and complexity..short .coms with easily mistyped domains.