Total Voter Turnout (approximate): 356,000

Percentage of total vote
24.5% Obama (D)
20.5% Edwards (D)
19.8% Clinton (D)
11.4% Huckabee (R)

Source: Group News Blog

And bear in mind that it takes a lot more time and energy to vote in the Democratic Iowa caucus than in the Republican one, which tends to dampen turnout.