Carbon Trading and Carbon Offsets are 2 different things
http://en.wikipedia.org/wiki/Carbon_offset
http://en.wikipedia.org/wiki/Carbon_trading
Offsetting is voluntary & is largly contracted by companies attempting to "green" their image. Lots of rich yuppies in big SUV's also buy "cabon offsets", and get a bumper sticker, to prove they're green at heart
Trading is a government-run program with emissions "credits" companies get dependant on a bunch of things.
both have their proponents & detractors. fwiw, my company has 20,000 employees, many of whom travel a LOT, and the company buys carbon offsets to cover 100% of it. the company also uses trees as part of the raw material, and grows something like 120% new trees for the ones used.
cheers