Currently, 50 bitcoins are mined approximately every 10 minutes.
Every 4 years, this amount will halve. December this year will be the first time. From then on, it's 25 BTC every 10 mins, etc.

So what's the theoretical maximum number of bitcoins ever to see the light of day:

4 years worth of 10 minute intervals is 4 × 365 × 24 × 60 / 10 = 210240 'mining hits' every 4 years.
The first 4 years it's 50 BTC per hit, next 4 years it's 25, then 12.5, etc.. so in total that's 210240 × 50 × (1+½+¼+...) = 210240 × 50 × 2 = roughly 21 million.

When will this be reached:

After N periods of 4 years, a fraction of 2^-N unmined bitcoins remains. This means that around the year 2040, 1-2^{(2040-2008)/4} ≈ 0.996 or about 99.6% of all bitcoins have been mined.