Core 2 Duo–based Macs ship with 802.11n-capable hardware, but to get the software to enable this you have to buy a new AirPort Extreme base station. According to iLounge, this is not because Apple’s trying to push base stations, but rather because they’re trying to comply with accounting rules from 2002’s Sarbanes-Oxley Act:
The company believes that if it sells a product, then later adds a feature to that product, it can be held liable for improper accounting if it recognizes revenue from the product at the time of sale, given that it hasn’t finished delivering the product at that point.
In other words, it may be illegal for a public, US-based company to issue a free software update that adds features. I’m not a lawyer, and I haven’t read the Act, so I have no comment on whether this legal interpretation is correct.