Go away, I'm sleeping.
We currently have a policy that equipment should be turned off when you're away from your desk for a period which appears to work well enough for a small institution - certainly staff machines/monitors aren't left on overnight and many are turned off over lunchtimes. Our 24 hour student lab have settings that mean they should go to sleep when machines are idle and not logged in.
However, I still have doubts as to how reliably computers go to sleep and resume properly if they are in use. Network connections will be broken so our accounts staff will all lose their ssh connections to the server. It may also be due to Netware, but we've found that if the network link to the server breaks, the drive mappings don't always get re-established and documents left open can't easily be saved since we deny users write permission to the local hard disk. This may work better in an Active Directory environment. Time will tell.
Even at home on a modern Vista machine, if I hibernate about 1 time in 10 it hangs on going into sleep and everything is lost on restart. And it never actually turns off either so you're still left using power. That problem is probably down to something I'm running, or a driver that is interfering with the shutdown (although can't see what since the display is the first thing to get deactivated). But these potential issues just make me wary of pushing that sort of unreliability out onto our staff without sufficient testing that I don't currently have time to do.
Finally, I've just measured the power consumption of a Dell Optiplex, the same as we have in our student computer lab. When sitting idle, it draws 68-75W. When I put it into standby, it is still using 58W. Therefore putting idle computers to sleep doesn't appear to save as much power as might be expected - certainly that's no reason not to do so since every little does help.
But perhaps another answer is having a policy of turning off rather than passing the buck and making it an IT problem. It uses even less power (around 2W) and there's no worry about returning to the desk to find missing work since you will have saved it before turning off the machine. Sure there's the time it then takes to boot back up, virus check etc - and the added power consumption of the boot process but managers need to weigh up the implications in cost, time and greenness and not just rely, as usual, on IT staff producing a magic bullet that achieves everything.