I tried one of these out this weekend:

Crank Brothers Adjustable Seat Post:
http://www.crankbrothers.com/seatposts.php

Super handy for a quick transistion from climbing to bombing downhill, but worth $200? The adjustment lever mounts on the handlebars, which makes things a little clustered. There's a seat-mounted lever(similar to an office chair) that seems more attractive though.