That would be a very good idea and probably a considerable source of revenue for project-gc.
If you do decide to implement this feature, _please_ consider to give it the same benefits that the referral system has today. Otherwise the feature will be somewhat undercut by the fact that you get a total of three months (1 for you, 2 for your friend = 3 total) _less_ by purchasing the membership as a gift instead of gifting the money and have the recipient purchase it using your referral code. If project-gc feels like rewarding getting new people recruited, who does the actual payment for it should be irrelevant - or am I missing something ?
EDIT: If you do decide to implement this feature, make sure that you provide the buyer with a nice printable-and-then-enclosable-in-an-envelope-under-a-tree-giftable PDF file :)