I have one of these to turn my am bike into an xc. Super long and alot more clearance than the thompson for diff seat positions. Same type of clamp but half the price and slightly heavier. You could get that signature nose diver seat thing going with the APX foshizzle.
I've heard some bad reviews about that I-Beam post and you can't use any of your old seats with it.
the APX works with just about any size seat rails from bmx seats to xc.
Sette APX Alloy Seatpost - all sizes- $24
http://www.pricepoint.com/detail/12719-275_SETAA4-3-Parts-68-Seatposts/Clamps/Sette-APX-Alloy-Seatpost.htmthrow in a fit bmx seat for $25 and you have a light combo that works great and looks better than any of those WTB XC weenie seats for 1/4 the price.