The Earth System Research Laboratory has the longest running system for measuring carbon dioxide levels in the atmosphere at Mauna Lua, Hawaii.

Here is their most recently released data on atmospheric co2 trends:

Globally-averaged co2 levels are generally consistent with this trend picture both in shape and parts-per-million estimates lending credence to the claim that we share a common atmosphere.

If you want to use a number to characterize the best current estimate of the level of co2 in the atmosphere you could use 385 ppm (grist.org is using this estimated value). What do you think it will be at the end of this year? Next year at the same time? 10 years from now? 100 years from now?