Just off the phone with Apple, which says Mac owners with 802.11n-capable hardware will be charged a $1.99 fee: A spokesperson with Apple provided me with a response that explains that the updater will be available for purchase from their online store at a "nominal fee" in order "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."
The company will eventually fully enable 802.11n on all Macs with appropriate hardware that they ship. The spokesperson couldn't yet provide a timetable for when that switchover would happen. If you purchase a Mac with Draft N chips between now and then, it's possible you would need to pay the $1.99, but, frankly, you'll just find someone with the enabler, won't you?
The enabler will be included on the new AirPort Extreme Base Station's CD.