Someone ran a similar thread a few months back and got a bunch of replies. The problem is, many newer road bikes don't follow traditional geomentry, with a top tube that is horizontal to the ground, and the seatpost clamp just above the top tube. So, a guy would post to say "My inseam is 32 inches, and I ride a size 52 bike." Quite unlikely that guy was riding a tradional bike, as his correct frame size (if his slack's inseam is 32 inches) is more likely to be around size 56 (for racing) or size 60 (for touring or commuting).

The second problem in the thread was that some people were referring to inseam as being the inseam on their slacks (32 inches) and the next guy, who also wears 32 inch slacks, was using inseam to mean leg length from the pubic bone done to the floor (35 inches). Without a shared understanding of what "inseam" means, the posted measurements were not very helpful.

The Rivendell website is helpful in relating leg length to frame size. They explain how to measure total leg length from the public bone to the floor. And, they have a chart suggesting frames sizes. Rivendell's chart assumes that their customers are not buying their bikes to race, and so their suggested sizes are about an inch higher than what many bike shops would suggest (many bike shop clerks fit bikes as if every customer will be racing in the Tour de France next July).