The Michigan GOP has a useful map of Mitt Romney's bullet-dodging win in Michigan which shows how the regions of the state played out in a pretty stark way. These exit polls speak to this as well - note that Romney's supporters were generally older, wealthier, better educated, and less conservative than Santorum's. As the WSJ notes:
Mitt Romney won the overall vote by more than 22,000 votes. But Michigan awards 28 of its 30 delegates based on the winners in individual congressional districts. So while Rick Santorum was the loser, it wasn’t a total loss for his campaign because he won seven districts – including districts 1,2,3,4, 6 and 7 in the northern, western and southern parts of the state. Mr. Santorum also won in district 13 – which meanders from Detroit’s economically depressed east side to blue collar suburbs of Lincoln Park and Westland. Mr. Romney won everywhere else in Metro Detroit – from the affluent communities of Oakland County to the industrial suburbs of Macomb County and up into the state’s rural “Thumb” area. He also took district 12 which includes Ann Arbor, home to the University of Michigan.
If exported to the fall, it's another sign that Romney's strategy for the general election will almost certainly involve targeting more affluent suburbanites, not the more economically depressed. This dovetails with Obama's own decision in that regard: both are deciding whether to target upper class suburban professionals, or play to the concerns of the white working class. Romney's appeal is clearly primarily with the former, with Obama choosing the latter - and the more aggressively populist class warfare message which comes with it.