Reading Bendis’ last issue of Iron Man I got one of those nice little surprises that I really enjoy, a Marvel character making a DC comics reference. Tony turns off a zombie killing contingency with the code phrase “the infinity killed Sue Dibny” You can also find Marvel characters talking about someone’s Kyptonite. Any other examples?
I recall an issue of Mark Waid’s Daredevil where a kid in a school bus sees him swing by and shouts, “IT’S BATMAN!”
In Thor #341, during the classic Walt Simonson run, Thor literally bumps into Clark Kent and Lois Lane.
The big one is probably the Squadron Supreme who are thinly veiled analogs for the Justice League (Hyperion=Superman, Nighthawk=Batman etc).
Lois and Clark popping up on the borders of Marvel comics is a grand tradition. If you look close you can see them painted into several of the crowd scenes in Marvels.