Apparently, any Mac bought in the last year or so already has 802.11n built in, but requires a software upgrade. It is currently rumoured that in the US, at least, this upgrade will cost $4.99 .
This is where it gets confusing. Apparently, Enron are to blame. The US Government introduced rules to control Accounting in the wake of the Enron collapse.
These rules stipulate that if any product has a significant upgrade, then it has to make some money in the quarter that the upgrade is introduced in..
http://www.reghardware.co.uk/2007/01...fi_fee_update/
Now, I personally wouldn't count it as an upgrade, as the hardware for it is built into the Mac, and therefore has been bought (and realised revenue) already.
However, it's worth noting a couple of things. First, this is a US Law, so won't apply here. Second, it's still a rumour.