Can Galaxy S use 802.11n for tethering?
I have read, that even though the GT-i9000 claims to support 802.11n, it does not. I even read that if your router is set to 802.11n "only" mode, the SGS will not connect to the access point. This is why you see the device using 802.11g. 

I don't think you can change the channel that is used with the "built in" tether, but some of the 3rd party apps look like they let you change the channel.