Originally posted by hawk220 
you can ask about 1200 people, (truly random) and get a projection of all 280 million
any statistical analysis majors know the exact number? 
 You should be able to achieve a generalizable sample with as few as 40 respondents, but you'll have a large margin of error.  Basically, the more people you interview, the smaller the margin of error with diminishing returns.  For most polls, this is a cost consideration; while it's possible to achieve a +/- 1% margin of error (or less), you'll need to interview thousands of people to do so.  If you're willing to deal with +/- 3% error, you'll only need around 1000 people. *
As it requires magnitudes more respondents to achieve the +/- 1% margin of error, from a cost perspective it is 
much more attractive to go with fewer respondents and a slightly higher margin of error.
All of this assumes truly random sampling.
* These numbers are just off of the top of my head.  I can provide the actual sampling numbers required if you're really interested in that.
-- Todd/Leviathn