Here's a printable pulley for GT2 belts with a 2mm pitch.
Updated using drofart's excellent parametric pulley design here; http://www.thingiverse.com/thing:16627/
Probably useful for anyone who doesn't want to customise the pulleys in openscad.
I'd recommend the 18 tooth pulley over the 17 tooth pulley, since I find it's harder for the belt to engage the smaller pulley perfectly. Might just be me though, let me know if you have any success with it.
Fewer teeth (More resolution) and smaller hub with two nut traps.
Personally I print my pulleys at 100% fill. I'd reccommend using SFACT with stretch on to offset the width of your extruded plastic in order to get the printed tooth profile more accurately. I'd recommend a lower height over width radio than normal in order to get a nice consistent perimeter edge.
Printed with a Prusa Mendel with T5 belts and printed pulleys, so it is possible to print an upgrade to the GT2 pulley. Your printer needs to be very well calibrated in order to print circles which are as true as possible; oval pulleys cause all sorts of problems with backlash.