Oakland is very wealthy, and leans libertarian (economically conservative, socially liberal). Macomb is more working class, and leans populist (economically liberal, socially conservative). Thus, as social issues have become more important and economic issues less so, these counties are slowly switching allegiances from what they once were.
Macomb has voted Republican in most Presidential elections since the '60's, with the exception of '96 and '00. A lot of the county seems to have gentrified (the south of the county is still mostly Democratic though). A bit like Anoka county, MN actually.
Oakland went to Kerry by just under 3000 votes because of a decent turnout in Pontiac and just enough socially liberal suburbanites worried by Bush's wedge issue policies (a bit of a freak result actually).
True. Interestingly, suburban Detroit swung away from Kerry this year. Were it not for non-Detriot metro Democratic areas, Bush would have won Michigan.