I think people are overly paranoid about this. An employer isn't going to be shocked by the idea that you have photos of you at parties. Most big firms have a big social culture anyway. There are some things which will create a bad impression, eg a lad at my uni, who was applying for Teach First, his profile picture was of him kneeling down sucking on a bottle which was strategically being held by his mate at crotch level, and his interests were "your Mum" and "raping orphans". That probably didn't enhance his chances if they saw it!
I have my facebook pretty private but to be honest it wouldn't bother me too much if an employer had full access, what they would discover from it is where I went to uni, where I went travelling and the pictures of the things I've done, and some pictures from a good extra curricular I've got, so actually a bit of stalking would verify that I haven't been making it up. There's nothing offensive written on my wall, if they want to browse through it they would just see a bit of chat about football and some posted news articles about economic issues that I've been discussing with mates.
I reckon with most people what you need to watch for, more than facebook (where most people have high privacy settings anyway) is what a google search of your name reveals. Eg, if you are applying for a job with the Civil Service, and you are telling them on your application form that you are non-aligned politically, and a google search brings up stuff you've been posting on socialist websites, stuff about you as the socialist worker organiser on campus and so on, your blog discussing direct action against the state, then its going to be an issue....