my mates got a 29.6mm thomson seatpost (its been cut to length, size is via vernier calipers) as its machined i'm guessing that its supposed to be 29.6mm, but what frames take that size?
Mine might not be standard though (it's had a replacement seat tube) - it's ok with a 27.2 & a shim that's supposed to go to 29.4, but doesn't clamp properly on a 29.4 post. Bizarre. Really need to get a proper measurement taken of it!