I could imagine people coming into that office on a Monday morning wondering why it's so damn hot in there. I think the drone of 600 overheating PC's would likely be a dead giveaway. 

If you mine with poclbm or GUIminer try using the -f "frames" switch and a high number. Something like -f100 or -f200 on each of your GPU workers should free up enough cycles to let you use the computer like you're not even mining at all. Just keep trying a higher number until you get the effect you want. The higher you go, the lower mH you'll get though. So it's a trade off. I think there is a similar switch for Phoenix but don't remember it.

I solo mined SC for 4 days on a 2+ Gh rig, and didn't hit a single block when the calculator said I should hit every 8-9 hours. I had a strong feeling my hashes were going into someone else's pocket.

It's all about cooling. If you can keep it cool, and running stable while mining it won't hurt your GPU's any more than intense gaming would. Long gone are the days where a GPU fan is a 5 cent piece of junk prone to failure in 6 months. Most of them are fairly heavy duty now. If you mine 24/7 be sure that once a month you clean all the fans / filters in the case especially if you have pets. Clogged fans will do more to blow up your PC than anything else. That goes for rigs with or without a case. You'd be surprised how many dead bugs came out of my CPU heat sink after 1 month of open box mining.