Tom Holbrook has begun posting some data on the 2008 election, and will continue to do so. Below is a nice finding from his first post. It compares the percent of voters who were under 30 years old in selected 2008 and 2004 primaries.
In almost all of these states, young people constitute a larger fraction of voters in the 2008 primary, relative to 2004. Tom has similar data (though not necessarily similar findings) for women, African-Americans, and independents.
Check out his blog, and check back for more updates as the campaign unfolds.