Lee's cameo aside, viewers who were actually trying to keep up with Dr. Selvig's ramblings (or simply looking at the chalkboard behind him) were treated to not just one, but several easter eggs that could have serious ramifications for the future of the Marvel movie universe.
For starters, the brief phrase written over Selvig's right shoulder: 616 Universe. For those unfamiliar with the comic book source material, Marvel Comics, like DC, has accepted that the majority of their superheroes exist in just one of several parallel universes. The name given to the one that Earthly audiences are used to seeing? Earth-616.
Although the term is disliked by several Marvel executives - since Marvel has never explored more than a handful of 'other Earths' - its appearance here implies that Dr. Selvig is aware of Earth's parallel incarnations.