Props to http://tzikeh.livejournal.com/448306.html
In summary, many states offer no accountability for their voter purge procedures and are inaccurately purging completely valid people from the rolls. Some continue purges beyond the 90day boundary to a Federal Election.
If you go to the polls and are "not a registered voter", then demand a "Provisional Ballot" as a right guaranteed by the Help America Vote Act of 2002.
Questions or complaints (in TX only):
P.O. Box 12060
Austin, Texas 78711-2060
http://www.sos.state.tx.us or http://www.votexas.org
For other states, contact your secretary of state to find out your rights and resources for national elections.