Greg Palast describes the way Republicans have used "Caging Lists" to challenge voters and suppress the vote of minorities and the poor. The trick is that if they discourage a hundred Black voters from voting, they may have lost 10 votes but their opponents will have lost 90.
Palast's article is a bit dated, having been published June 23, 2006. The VRA was reauthorized and signed into law by Bush on July 27, 2006 for another 25 years, so it remains effective for those jurisdictions it previously applied to. That means that it remains illegal to identify voters by race in order to challenge their right to vote.
That includes targeting districts with a high concentration of African-American voters, sending out registered letters to the registered voters in that district, then challenging those who do not respond or who do not pick up their letters. Registered letters are often used by bill collectors, so people behind on their bills rarely pick up registered letters from unknown addresses. This is not a reliable way to find who has moved, nor is that the purpose. It is just a way of challenging voters who are likely to vote Democratic.
Such practices are going to be quite common in November 2008, illegal or not. Unfortunately, such practices are not nearly as effective for likely Republican voters. It works when used on those minority voters who the Republicans are clearly out to get, such as African-Americans, immigrants, Hispanics, and homosexuals. Except for Republicans in general, there are no such minority groups actively threatened by Democrats.