Probably in for a bunch of speculation here, but I'll bite.
I read somewhere once (owners manual for something perhaps) to let the vehicle warm up for 15 seconds.
I would assume this probably applies to fuel injected vehicles, because idle typically goes to normal pretty darn quick. More than anything anymore, they are more likely afraid some idiot will not completely depress the brake and inch forward into their garage wall, and sue.
Anyways, I don't think a long warmup is probably necessary. It may help, but I have a feeling proper maintenance (oil, filter changes) is the key to longevity, not extended warmup. Lots of the internal lubrication of the engine is by oil being slung, idling is typically claimed as being hard on the engine. Besides. Look at the way most people drive their vehicles that are lasting at least 150,000 miles. Hop in, throw it in gear, and go. (yes, newer engines are different, but IMO not THAT different)
I like the 15 second rule, makes me feel good. haha. Not that long to wait, and as long as you aren't lugging the transmission down with an overly high idle (also probably not really a problem) I say move out. /forums/images/graemlins/smile.gif