Buy it now if you're not interested into having the latest GPU or an SSDs option from Apple.
The CPU can't get any better (see Intel roadmap), USB 3.0 will happen next year (thanks intel), RAM can't get any better (16gb), displays can't get any better (IPS+LED already), design will stay the same at least till fall 2011, so what are you wating for? Nothing....
Plus, if the update happens in October, you loose 5 months of iMac experience and you will feel stupid when, after 2-3 months, the lightpeak/USB3.0/SATA6gbps/SandyBridge revolution happens.
101% agree..