I've never had a problem with leaving the computer in Sleep mode while transporting it. The latch on my machine (PB G4 1GHz) is a little flaky and will sometimes release the lid which causes the computer to wake up, however my system profiler is set to put the system back to sleep within 7 minutes of inactivity so it has never been a problem. (never even gets warm)
Don't worry about it. Not a problem.
Sopranino