Well, clearly there can't be an exact record of how many PCs have been sold ever, so there's no way to reward the guy who buys the billionth PC (it's probably already happened, actually). 1 billion is an extremely rough estimate...probably based on numbers of processors manufactured, or something like that (that's the best way I can think of to figure it out).
Also, I think 1/6 people having a PC is quite high. Just because a billion pcs have been sold, doesn't mean there's a billion people using a pc. Consider the stack of ~20 computers at my work, or any corporate computer, for that matter. Additionally, there are 6 computers in my home, for 3 people residing here. Let me see if I can find the US Census information about computer usage (for the US) and attach it. [back] Alright, I couldn't find the file (I think I now remember deleting it), but the page should be available somewhere on
here (man, and I thought works cited pages were completely useless...I used that source on my Mac vs. PC paper, available in the Mac Parody and Are Mac Users Smarter threads).
If someone has lots of time, or is really interested, feel free to find the information (for the US) on that page, and report back. Remember, though, that the computer usage in the US is probably the highest in the world.