2003 Specialized Hardrock, 2004 LOOK KG386i, 2005 Iron Horse Warrior Expert, 2009 Pedal Force CX1
I don't see why any seatpost could not be shortened. The only thing you need to remember is to mark the cut post at the same length from the end as the old post with a minimum insertion line. If it's a non-round cross section tube (like my Ritchey WCS posts) or a carbon fiber seatpost of any style, you'll want to use a fine tooth hacksaw blade for cutting it. Read up on cutting steerer tubes for info on how to make sure you get a nice cut if your seatpost is carbon fiber.