Didn't apple factory overclock the g5 to get the 2.7's? Surely something would be possible in the near future, assuming they'd want to. Personally, I see no need for a 2.8ghz iMac. If you are really missing that sort of processing power, you should be looking at a mac pro. Apple clearing tells us that as consumers (and, therefore, those who would buy an iMac) we don't need gobs of power, we need a small quiet art piece.
I see a 2.8Ghz iMac being only slightly more likely as a resurgence of the cube. A lot of the arguments against the cube would apply to this as well. It would eat into the Mac Pro market share. What we already have is "good enough." It would complicate the product spread. Not to mention Intel doesn't presently produce a 2.8Ghz chip unless you go back to the aging netburst architecture - which, I believe, is very close to being phased out.