I don't do ACL rehab, but my understanding is that relatively early mobilization tends to produce better functional outcomes - so i don't know about immobilizing the knee for 3 months, seems kinda outdated...
again it's not really my area (largely bec most ACL post-op stuff is protocol driven, and I find that sort of thing rather dull), but searching the net should give u a good idea about what's going on these days over here...if u find anything specific (e.g. - ACL rehab protocols) and have any questions about it, let me know
mayb some of the other physio / PT guys (Kwaichang and R) on here have something better to offer?