I think the friends with ex's thing IS possible, but only after a period of zero contact. I see people who try and be best friends the day after their relationship has broken up and it doesn't work.
I'm 'in contact' (msn&myspace) with one of my exs and friends with another - that is, I probably wouldn't call him up, but we'll sit and talk at college from time to time, and I'd go out in a group with him, just not 'just us'.
I think it's nice to be friends with exs if both of you feel the same way about it - that is, that you just want friendship. Otherwise you're doomed - but that's true of any friendship..