I have a seatpost rack purchased from Performance. The max load is 25lbs. You'll need to torque the mounting bolts pretty tight to get a solid non-squeaking fit, so I wouldn't recommend it for a carbon post. Coupled with a cargo net, it's great for running to the store or to carry a sweater & lock.
Here's a pic
http://i218.photobucket.com/albums/c...SC00003c-1.jpg