Okay it looks like it's time for me to eat some crow. Back in the Valentine's Day Ask TUAW, somebody asked if it would be possible to upgrade the 802.11g wireless card in iMacs and MacBooks to be compatible with the new 802.11n wireless standard that Apple just recently released and I said probably not. Well it turns out I was wrong, though I should get partial credit because I did actually mention QuickerTek in my response. Today QuickerTek announced the availability of a wireless N card that fits in the Airport card slot of "modern MacBooks and MacBook Pros" as well as "modern iMacs." They say: "Since this wireless upgrade uses standard parts, no drivers or other changes need to be made, nor are additional steps required to use the higher speed 'n' upgrade."
I just called QuickerTek and they confirmed that this upgrade only requires removing the old card from your Core Duo MacBook (Pro) or iMac, connecting the antennas, and running Apple's enabler upgrade software (which, as we mentioned before, will run you $2).
QuickerTek offers the bare card for customer installation at $149 or they'll put it in for you for $199 if you send them your Mac.
Update: We previously mentioned doing this yourself with a Mac Pro upgrade kit (which is substantially cheaper), though be warned that Apple claims this will violate your warranty.