Apple will be charging a nominal fee to users of a particular upgrade:
The fee for the software is required in order for Apple to comply with generally accepted accounting principles for revenue recognition, which generally require that we charge for significant feature enhancements, such as 802.11n, when added to previously purchased products.
There is more coverage of the reason for this at the Register: blame legal changes in the wake of the Enron scandal.
If this interpretation holds true, does this mean the end of free software upgrades?