Unfortunately the iMacs no longer have a set update schedule. If you look at the Mac Rumours Buyers Guide page you'll see the last few updates have been any time from about August through to April. (The last two were March and April, and the previous two were August and September.)
Although it's possible Apple will update the iMac in September, I don't think it's likely. In September they are already updating the Mac OS, plus the rumours of updating the iMacs, iPods, and introducing the "iTablet" - that's a lot of releases in one month, so I wouldn't be placing bets on them.
With Snow Leopard being released in September it's probably best to at least wait for that - you'll save the small upgrade fee and the hassle of installing it. (I haven't checked, but I believe the Mac OS update fee is actually a rebate, which means you'll have to pay full price and then wait for the refund to arrive. 🙁)
Usually the best time to buy is a month or two after Apple has released new hardware (or software for that matter). That way you get the newest machine and they've had time to work any bugs out - the problem of course is the stagered release of the computers and the Mac OS, but at least the Mac OS can be more easily owner-updated.