In 2004, Nevada Republicans had a small registration advantage: 434,239 to 429,808. By 2008, the Democrats had built a huge registration advantage: 625,134 to 513,629. These changes were reflected in the presidential voting, turning a small Republican edge of 21,000 votes in '04 to a blowout Democratic win of over 120,000 in '08.
Nevada has an interesting system of differentiating between 'active' registered voters and 'inactive' registered voters. Essentially, 'inactive' registered voters are those in which records indicate that they may have moved. They can still vote if they can show that they are legally able to do so, but I would think that such voters would be significantly less likely (though to what extent I don't know) to actually vote. I believe that, like increasing (or decreasing) voter registration, it is a proxy for enthusiasm.
In 2008 almost all of the Democrats' advantage was in 'active' voters, with about 101,000 of their advantage coming from them and only about a 10,000 edge in 'inactive' voters. This year, however, while the total Democratic advantage is only slightly smaller, their advantage in 'active' registered voters has been almost halved to 55,700. About half of it is now in 'inactive' voters. There are 127,000 'inactive' Democratic registered voters compared to only 76,000 'inactive' Republican voters.
One final issue is the growth of the Independent American Party. When you look at their site, it is clear that they are essentially Tea Partiers. If you look at only the active registered voters as of August 2012, Republicans, when combined with the IAP, are essentially tied with the Democrats.
Nevada looks to me to be in play as of the August data. Obama was clearly in much worse shape than he was in 2008 (though of course he can afford to lose quite a bit, considering the blow-out double digit win).