If what you say is true, then there must have been a HUGE amount of crossover McCain voters also voting for Brown, since Obama only won the county by 5 pts. Also, this is the first time since 1964 that a Democrat has won Nevada county.
From the county by county vote it appears that yes, many McCain voters also voted for Brown. Remember, if all you knew about Charlie Brown was what you heard on the radio or saw in his ads then you would assume that he was an arch-conservative Republican. He literally had ads ending with something like, “Charlie Brown, standing up for conservative family values” (paraphrased, but close). Also, the Democrat’s house campaign committee put a lot of money in to running negative ads against Tom in this campaign. I think they did some damage.
However, I stand by my statement that Nevada County is more liberal than the rest of the 4th, and the numbers there were WAY out of skew with the rest of the district. Someone made a comment about ACORN - my thought was simply this - if you want to look for Shenanigans in the 4th the place to look is Nevada County. I just checked the final numbers - Charlie Brown out polled Mr. Obama by over 2,000 votes out of only 43,000 cast.