I looked up the dates for past NASCAR week races in LV and came up with this since 2001:
March 4th, 2001
March 3rd, 2002
March 2nd, 2003
March 7th, 2004
March 13th, 2005
March 12th, 2006
March 11th, 2007
So it appears that the last 3 races have been the second weekend of March but the first 4 appear to have been the first weekend of March.
Nothing is for certain but, I'd still bet on the second weekend of March 2008 which would put the race on March 9th, 2008.
Perhaps they found the crowds were still a little to thin so close to Feb. for their tastes or perhaps fan feed back indicated it needed to be later in the month. I'm not certain why it would have changed from the first weekend after three years to the second weekend but that appears to be what's happened.