I think by law all states have to allow absentee voting? It's just in a bunch they make you supply an 'approved excuse'.
http://www.zwire.com/site/news.cfm?BRD=1829&dept_id=107421&newsid=12978669&PAG=461&rfi=9
According to a recent article in the Wall Street Journal, at least 24 states allow anyone to mail in an absentee ballot, for any reason. These include most of the states west of the Mississippi River, including Iowa, as well as North Carolina and the battleground state of Florida. At least 16 states also open the doors to actual polling places for several days before Election Day, as does Iowa. Allamakee County residents can vote at the Auditor's office in the courthouse in Waukon beginning September 23 for the November 2 election. The Auditor's office will be open October 23 from 8 a.m. to 5 p.m. (the last day for voter registration) and Saturday, October 30, from 8 a.m. until 4 p.m.
Twenty-six states plus the District of Columbia still require voters to provide a reason from a list of approved excused absences (such as being disabled, on vacation, stationed overseas) in order to request and receive an absentee ballot. A new rule in Virginia grants a mail-in ballot to anyone who says they'll be at work and commuting for more than eleven hours on Election Day.