All of the Core 2 Duo (except 17" 1.83GHz iMac) and Xeon Macs have been shipping with the draft-N hardware. People have been using it in Windows, and Apple should be pushing out an OSX driver soon.
802.11n has been an ongoing saga for the working group... it was originally going to be finalized in like 2005, but so many companies with competing visions have gotten onboard that the final spec won't be released until at least April 2008, and I have a hunch that date will slip again.