I'm on Orange, and have been since April, and after my first month I've never even approached the 500MB (I usually end up around the 250/300 mark). However my friend has gone over (on the same Dolphin plan that I'm on), and they apparently put you on a daily rate for every day you use data once you're over your limit, and it's like 60p a day. Of course that has a fair use too, but you'd have to be really going for it to go over your own limit AND the daily limit too. I think you'd have to be really stupid to rack up a massive bill, even if you were on the daily rate all month that's only an extra £18.
I thought it was too good to be true, but check it out for yourself:
Phone plan: pay monthly full details - Canary, Dolphin, Panther, Racoon
Having said that I recommend you do download 3G Watchdog and keep an eye on it, if only for your own peace of mind.