Does a carrier know I am tethering via Wifi Hotspot? I am using my phone's built in Wifi Hotspot to share wifi with my laptop and my other devices.  I am using android 2.3 (Gingerbread), and the phone I have is the HTC Desire Z. 

Questions

Is it possible for a carrier to detect that I am using "Wifi Hotspot" and sharing my mobile data connection with other devices?

Furthermore, is Wifi Hotspot a feature that most android devices have? and is it a feature that can be disabled by the carrier (on a phone-by-phone basis)?
It is not detectable, but using tethering on Android with an second device leaves a lot of traces:


User-Agent Headers within HTTP that come from non-mobile browser
Increased Traffic consumption
Connection to services that are not available on Android/Smartphone devices (e.g. World of Warcraft)


and many more I can't think of atm.

But I have never heard of a carrier that goes for a trial, even if it's a violation of services from most carrier contracts. IMHO Mobile carriers have other problems at the moment, like overall increased mobile data traffic because of the smartphone boom. These is currently handled by soft limits of data traffic, that when reached only allow traffic over 2G connections. 
Or, to answer your second question, if you buy a smartphone from some carriers, the build in tethering function is locked and not usable.