According to this article as many as a quarter of the votes this election cycle may be absentee votes. The concerns about the validity of these votes are specific and legitimate. Take a look at AlterNet: Election 2004: Early Voting Could Be Perilous:

"Some states actually allow political parties to collect and turn in the applications. In seven states, parties can even collect and return the signed ballots.

You can't police absentee voting as you can voting at the polls, where the rule is the secret ballot, that mainstay of democratic elections. Electioneering and any other attempts to sway or intimidate voters are prohibited at the polls.

With absentee voting, the possibilities for the intimidation of voters and other electoral mischief are many. Certainly, forgery is always possible. Even if the signatures on absentee ballots are legitimate, there's no guarantee that someone else didn't do the actual voting or didn't unduly pressure the signers - or perhaps bribe them."