I am sort of in the same boat. Want to get an iMac or 'maybe' a Mac Pro for the quad / octa core and expandability. I doubt there will be an iMac update in June as the other person already mention usually that is not their product cycle (and way too soon just after a refresh), iMac updates could come in fall / end of year, which for me is still not worth it to buy it now if there really would be one coming this fall or end of year (given how small the last update was...)
Plus for me the chance of an iMac with a quad core proc is worth the wait. If you really need a machine now and need to get work done with it, then I say you can still get the latest iMac right now.. if you really need it. But if you can hold out, maybe wait and see (knowing!! there might not be an update that soon OR there wouldn't be a big update like quad core.. etc either if there was one)