A conundrum. Others have variations of this.
One approach is to use a single black pixel as a screen saver, then tile it (Mac OSX Hints). I like my photo screensaver though, and switching screensavers night and day is a nuisance. Brightness Control is great, but the screen is not all that dark, and I think when the screensaver (photo show) turns on BC turns off.
Now I'm using Basic Black 2.0 (donationware, but I don't do PayPal. Wish Google Checkout did person-to-person payment). Of course that has the same issues as the single pixel problem. Except it occurred to me that I could make it the screensaver on the child account, and switching users is easy. So that's the compromise for the moment ...
Apple should have done something about this (hmpphh), but doubt they'll bother with it for their older machines.
Update 8/11/2007: Dockables screen sleep puts my iMac screen to sleep! I didn't think that was possible.
Update 5/22/2009: In 10.5 (Leopard) Apple added an undocumented screen off shortcut key.