HardMac is reporting that the newest iMacs released this week support up to 450 Mbit/sec Wi-Fi, following in the footsteps of the latest MacBook Pros. To accomplish this, Apple added a third antenna to the machines. Each antenna is capable of a data rate of 150 Mbit/sec. Combined, the three antennas achieve the 450 Mbit/sec speeds using the multiple-input/multiple-output (MIMO) wireless standard.
MacRumors notes that in order to actually take advantage of the 450 Mbit/sec speed, you'll need to have a compatible wireless router, such as the latest AirPort Extreme or TimeCapsule, and be sure the settings are configured to use both the 5 GHz band and also the use of wide channels.
UPDATE: As noted by AppleInsider, the newest iMacs and MacBook Pros must be running a developer build of Lion to take advantage of the 450 Mbit/sec speeds. Non-devs will need to wait until you update your machines to Mac OS X 10.7 Lion when it is released later this summer.