I tried a Belking "pre"N wireless router with my Toshina Tecr A9 laptop which has a 802.11n card.
If I changed settings, channels, etc, I would get the higher connectivity (130 Mbps or so) for a few minutes, but it would eventually fall back to the 802.11g (54 Mbps) speed.
I am in the same room as the router, so interference shouldn't be the issue.
I then tried the same with an Apple AirPort Extreme. Same issue.
Since there are other computers connecting at the 802.11g speed because of their hardware, does that knock all connection speeds down to the slower rate?