That's not entirely true. There are some routers that scale the speed only in regard to b/g devices but leave n capability intact when accessing the router or accessing other n devices.
So if you have three computers, two of which have 802.11n and one that has 802.11g, you would experience n speeds when transferring files between the 802.11n computers, but the speed would scale down to g speeds when interacting with the 802.11g computer.
Again, not all routers will do this. Some will scale all of the computers down to the "slowest" adapter on the network. However, there are those that do what I mentioned in the previous paragraph, such as my D-Link DIR-655