They're popping up all over the place now. Frames that have a seat tube that extends through the top tube with a small telescoping extension to dial in the perfect saddle height. The latest is the Look 595 (ridden by Thor in the TDF). Time has been doing this for a while too, and personally I'm fond of this design. The first thing that jumps out at me, as I look at two frames in my basement with the seatpost welded to the frame is that it will eliminate this problem. Besides, how much do you really need to adjust the seat post anyway? It will also provide for a stiffer coupling without the added butting and weight of the conventional design, not to mention the ability to use a more shaped seat tube for aerodyanmics and strength. An obvious con is being limited as far as set-back that you can purchase with an aftermarket seatpost. Be careful when cutting the seat tube too. This isn't a $300 fork you're fooling around with.