Radley Balko's got an interesting series of posts on the subject. My take. In Central Illinois, say Bloomington, Pekin, Morton or Peoria (Tazewell, Woodford and Peoria counties), where I am from, most folks say "soda" but you won't get too many funny looks if you ask for "pop" though. However, in the more rural towns "pop" is not uncommon, if not the standard choice. Both seem to work just fine in Central Illinois, with "soda" being the standard in the larger cities. The further south you go in Illinois, the more folks will say "pop" opposed to "soda". Personally, I don't care what you call it, so long as you give me my Mountain Dew.
