So Barack Obama has an aunt who's in the country illegally. Obama says he didn't know aunt's illegal status - Los Angeles Times
What's important about this story (to the McCain campaign) is not Obama's connection with illegal immigration. Few people who would otherwise have supported Obama are going to turn on him because an aunt he barely knew outstayed her visa.
What's important is that it reminds people of Obama's Other-ness. Reminding us that his aunt is Kenyan and has a name we can't pronounce--that's something that really can hurt Obama.