Yes have seen residents dating attendings...don't go in their bedrooms so don't really know who is sleeping with who.
One of the current chief residents is marrying one of the younger attendings, so assume they were dating at least last year, when she was a resident.
Male chief resident a couple of years ago was dating a med student seriously...he had been divorced a couple of years prior. I personally don't care what other people do, but I think it's silly to have it in the house staff manual that you can't date anyone not on your level of training, and then not enforce the rule, or selectively enforce it.
I personally think that residents dating med students is a bigger issue...or residents sleeping w/med students in a nonserious way. One of the male residents was caught in a call room w/female MSIII a couple of years ago, which I think is poor form...don't know if it was "his" team's med student, which would be worse than if she wasn't, IMHO.
At any rate, anyone who think's Grey's Anatomy is realistic has no clue what medicine is really like. Even more unrealistic than all the interns sleeping w/attendings on that show is the interns doing neurosurgery!!!