Well by using maths you could get something aproximative: Take the life expectancy of each country compared with it's WWII population, then add about ten years to get a fair age for each vet ( life expectancy + 10= 85 years old in 2007). You can then do this with the age pyramid and add more or less years. Population and age pyramids are easy to find on the net.
Then add each year for the major countries involved in the war and you'll get a total. Of course this will never be exact as the figure changes every day. But there still are millions around the world. Other figures are to be taken into account (the germans used Volksturm with kids and seniors for example, will you include Partisans and resistants ? some soldiers were from others nationalities etc... )
If you think it's too complicated, time will take care of these figures who unfortunately will dwindle dramatically within a few years. I however believe that each country has a figure availble at least for their own veterans, if you get those all you would have to do is to add them up.

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