Just in case anyone else was looking at these - http://cgi.ebay.com/ws/eBayISAPI.dll...m=390145108510
I bought a few rails off of eBay just to see if they were good for anything and got them yesterday...
They have ball bearings in them and they slide really nice, but there's quite a bit of play in them - they would be good if you designed a system that would keep them pushed apart or pulled together to take out the play
I can give more info if anyone's interested - I just thought I'd post my experience