According to Gallup:
Gallup’s latest update — based on interviewing conducted through Wednesday, Oct. 29 — shows that 21% of registered voters who plan to vote say they have already voted early or by absentee ballot. This percentage has been increasing steadily over the last two weeks. Another 12% of registered voters say they still plan on voting early, leaving about two-thirds of those who plan on voting who indicate they will actually vote on Election Day itself, next Tuesday.1
Only 32 states allow early no-excuse pre-election day in-person voting.2 In 2006, these states made up for 62% of registered voters.3.
If what Gallup says is true, more than half of the likely voters in these states will vote early (assume a reasonable margin of error in the numbers reported by the Census Bureau).
References:
1 http://www.gallup.com/poll/111673/Interest-High-One-Five-Already-Voted.aspx
2 http://www.earlyvoting.net/states/abslaws.php
3 http://www.census.gov/population/socdemo/voting/cps2006/tab04a.xls