is law only in Florida (700,000 ex-felons), Virginia (250,000), and Kentucky (150,000+). Mississippi and Alabama disenfranchise all ex-felons convicted of violent crimes. Other states have more complicated disenfranchisement rules, or they disenfranchise probationers and give exceedingly long probations (e.g. 30 years in Georgia) tacked on to sentences proper. The full picture nationally is a grotesque mess and travesty. All these states have various legal loopholes and processes to restore rights to people that they like, of course.
There's a decent map of things at
http://www.righttovote.org and a lot of paper and studies at The Sentencing Project website.
I've been interested in the whole problem since November 2000. Basically, Florida is the linchpin to the matter- other states will quickly follow suit once Florida caves. There have been a lot of lawsuits about it all over the years, with the worst sorts of jurisprudence and abuses on display by Republican-stacked federal courts. Johnson v Bush was probably the key lawsuit over the past few years; the court papers are on the Brennan Center's website.
Everything rests on the 1974 Supreme Court verdict/opinion in Richardson v Ramirez, which is the one remaining Rehnquist "accomplishment" still standing (other than re-legalization of execution i.e. "death penalty" in 1977). The RvR opinions have to be read to be believed; there are copies online.
One consequence of ex-felon disenfranchisement has been ability to manipulate voter registration and voter ID requirements in extremely abusive partisan ways. Katherine Harris did it infamously in Florida. It's a loophole through which all kinds of election abuses are perpetrated, and the abuses only end with elimination of the disenfranchisement laws altogether.