Is there any way to monitor 3G tethering usage. ie excluding regular 3G bandwidth. (This is for a UK GiffGaff plan, where normal use is unlimited, but tethering charged per GB.)
If you're rooted you could try Wireless Tether for Root Users.
More info and more recent versions here: https://code.google.com/p/android-wifi-tether/
Besides having way more options than the stock wifi tether application it also allows you to have a look at the bandwith used when tethering is active.