AT&T:

b = (int)5s/100; // where b is the number of bars, and s is signal strength

Apple:

b = (int)5s/100; if(b > 2) b += 2;

gruber linked to this on what Apple currently uses as their formula (neat graphic included): http://fscked.co.uk/post/754590440/this-infographic-hopefull...

AT&T's formula must be not so heavily weighted towards 5 bars as what Apple is currently using.