I think that if an employer *really* wants to find out about your Facebook, they might be able to, despite the Facebook privacy settings. If the employer is *really* determined to snoop like that, they'll investigate every way they can.
But I agree w/ camlan--I think most sensible employers:
1) don't really have time to invest beyond looking at your own posts;
2) are only concerned about what *you* put up--they recognize that we all have weirdos on our friends list.
And I agree w/ PastryGoddess--if you feel unfriending people is the only way to move forward, own it.
Or, you can fiddle around this way:
Contact them yourself to say, "I want to be sure we have each other's real-life contact info," if you want to continue to be in touch other ways. And then unfriend them.
You can always RE-friend them, if they notice (hey, blame it on Facebook--though that's the opposite of "owning" it) and send a new friend request. Or, you can wait until you're settled in the new job and send a friend request of your own. (You might discover that it's sort of nice to have a newsfeed free of those sort of posts!)