Will Biden endorse?
At this point, most endorsements are not about votes: they're about sending a signal to the other superdelegates that it's time to rally around the candidate who will finish with the most pledged delegates, the most states, and probably the most votes. This was clearly the point of Richardson's endorsement.
So while Joe Biden may not have a huge constituency in the party, he can still send a strong message by endorsing Barack Obama.
In fact, if Biden had a bigger constituency, it might make sense for him to wait. Most likely, nothing-- perhaps short of an extremely unlikely defeat in PA-- will get Clinton out of the race before all the states have voted. And some unaligned party heavyweights will need to step in and say, "This guy won." Gore, Edwards, and Nancy Pelosi strike me as having that clout.
But Biden is not quite at that level, it seems to me. And with Dodd and Richardson already having endorsed Obama, Biden's endorsement would send the message that all the second-tier 2008 candidates have decided Obama's the guy.
As an added bonus, it would bring back to people's minds the great debate moment when Obama defended Biden, despite his clumsy remarks about Obama being "clean" and "articulate." That was unity in action, and now seems like the best time to drive that message home once again.