^^ At one time that was true, but times have changed, and for the better.
I’m running an ASUS WL-500gP V2 wireless G router (dd-wrt) and have an ASUS RT-N13U B1 wireless N router (dd-wrt) patched to it running in AP mode (mixed wireless).
All my wireless G clients run at wireless G speeds, while all my wireless N clients run at wireless N speeds, even when both are connected at the same time. And it’s been tested w/ actual tools (e.g., NetCPS, iPerf). The ONLY time my wireless N devices take a hit (and only minor, ~5Mbps) is when the wireless G devices are “active”. But even so, my wireless N devices always run at wireless N speeds when they’re active. Like any wireless client, they only suffer in terms of throughput when more than one wireless client (G or N) is accessing the AP at the same time.
The days of wireless routers downgrading ALL clients to the lowest commonly shared speed simply because you have mixed clients is no longer a given. Of course, it may still be the case for older equipment.
To illustrate, I’ve attached a dump of the wireless status screen from the wireless N AP. The first two are wireless N clients and show link-speeds of 65/72.2Mbps and 58/65Mbps. The third is a wireless G client showing link-speeds of 48/36Mbps. Now granted, the link-speeds don’t tell us the actual throughput. But it does at least suggest that the router hasn’t AUTOMATICALLY downgraded the links to wireless G, and actual testing confirms it.
So the bottom line is, whether wireless G or N, associated or active, it just doesn’t matter. The router merely delivers whatever the wireless client is capable of handling.