Am I right in thinking, using a wireless-n router as a client bridge (without any gigabit ports) and connecting a device to it via a 100baseT ethernet port would completely cancel out the speed gains of using wireless-N?
For a wiresless-n client bridge do you really need a gigabit ethernet port before you see an advantage of wireless-n over wireless-g?
I would say gigabit wouldn't make much of a difference over fast ethernet.
Assuming your using a 300mbps 802.11n wireless connection, you probably will see little to no loss of bandwidth from the 10/100 wired connection. There are several factors as to why you do not receive the theoretical 300mbps. The bandwidth overhead in wireless and any RF interference are two factors that subtract from total useable bandwidth. 90-130mbps is my real life expectation of my 300mbps 802.11n router, whereas I would expect to see ~90mbps from 10/100 ethernet.
So if you go with gigabit you may see a few Mb more, but I wouldn't buy new gigabit equipped hardware for it.