The i7 is a configuration option. So go to the Apple store online, choose the iMac 27" and you'll have an option to choose the i5 or i7 processor.
Here's the difference explained by Apple:
[i]"The Intel Core i5 or i7 processor incorporates four processing cores onto one die, so data doesn't have to travel far to get from core to core. These "Nehalem" processors include the following features:
* 8MB of shared L3 cache, which boosts performance by keeping data and instructions in a fast-access cache that is available to all four processor cores.
* An integrated memory controller, which allows faster access to data stored in memory by connecting the memory controller directly to the processor, eliminating any middle man and significantly increasing memory bandwidth.
* Turbo Boost, a dynamic performance technology that automatically speeds up the cores in use when other cores aren't needed.
The Intel Core i7 processor also features Hyper-Threading technology, which allows two threads to run simultaneously on each processor core, providing eight virtual cores for increased performance."[/i]
Looks like there's a two week wait on the current 27" models...