One problem though...

Better start it off very conservatively, or you'll face carbon-credits-as-guaranteed-investment syndrome - people convinced that holding onto credits will put their 2 year old through college someday. Big positive feedback there, on a short timescale.

Squalish, credits are only good for a year, so no point in hoarding them. Tearing them up reduces the available CO2 though :) Also, unlike a tax, the cap is guaranteed.