In 2004, Republicans had just over a 4,000 edge in 'active' registered voters, and won by a similar margin of 10,000 votes. In 2008, much like Nevada, Democrats surged to a huge edge of 106,000 'active' registered voters. This lead to a 146,000 blow-out win for Obama in 2008.
So how is 2012 shaping up? Republicans currently have a 12,000 edge in active registered voters, bigger than they did when Bush won in 2004.
In spite of the Iowa poll averages, Romney looks to have the advantage this year.
I notice that Rasmussen's most recent poll shows a Romney edge of 3 points. This looks about right. Not from confirmation bias (looking for a poll that agrees with my opinion), but because that result agrees with the objective evidence from non-polling sources.