If this is for buying advice, I'd recommend an i5, it's the best bang for your buck and handles most tasks just as well as an i7, unless you are really pushing it hard.
If it's just out of interest, disregard my post.
True except for the MacBook Pros, the i7 is significantly faster and has twice the number of cores on the 15" and 17". In the iMacs the 2.8 i5 is closer to the i7 models.