A simple way for that shot be taken would be to expose properly for the background, then add some flash to illuminate the folks. I realise that doesen't quite help you now, but its something handy to know about.
For now... you could try to fix it using CS2, I guess, by making two copies of the same picture, one less exposed than the other, then layer them one on top of the other and erasing away one layer until the image looks right, but that would be fiddly. Shame not to try it though, any improvements would be good, its such a nice pic.
If you had CS4 you could use the alterations brush to very simply isolate the background and try to save whatever detail lurks in the RAW file...
Turning on Image Editing OK will get you some examples of how it could be done, as Nemo Man said above.