Riding the Powertap wheel on Tuesday before the spin session (yes, I do train!) I noticed a bit of vibration.  Looked down and the wheel was a bit out of true.  No probs ... Nath uses it on Tues night at Spin, I take it to the shop on Wednesday, have a look .. hrm, cracked rim!  It's the second Mavic Open Pro I've cracked now, the last one was about 3 years ago I think.  We had a spare and now the wheel's rebuilt and ready to use again.