Have, and would again if the circumstances were right.
I was crazy in love with this guy for the longest time, and we eventually ended up giving a relationship a try... but it just didn't work out for a variety of reasons. The breakup was amicable though, and we more or less became best friends afterwards. Anyways, about a year and a half after we'd broken up, he decided to introduce me to a friend of his that he thought I'd be good with. I wasn't really into the idea and kept flaking on the guy, so they ambushed me and showed up at my house one night. We ended up dating for over a year, and the three of us hung out together quite a bit. It was never really weird or anything while I was dating his friend, but the ex/best friend would occassionally tease us and make hilariously inappropriate comments. All good times.
The only regret is that I let the breakup with the new guy come between the friendship with the ex. The friend/ex had made me promise that nothing would change, but we ended up drifting apart anyway.