Read an interesting lead article in New Internationalist (July, 2006) which highlighted some of the greenwash that's occuring when tree planting is sold as an effective carbon offset.
The problem is that burning fossil fuels permanently adds CO2 to the carbon cycle while carbon storage in trees is strictly temporary. In the long run, it doesn't help. These scheme's also tend to fund mono-culture mega-plantations and exacerbate local land disputes. Or sometimes, the planted tree's just die in poorly run scheme's. More info at SinksWatch and FERN.
So if you do buy into carbon offsets, check where your funds really are going. For example, 20% of funds given to Climate Care goes to a reforestation project in Uganda.