I agree, his veep pick (if he does indeed get the nomination) will most likely be someone his hardcore supporters find "distasteful." In fact...
IMO, unless he offers HRC the VP (and if she doesn't want it she publicly turns it down) there will be problems bridging the large chasm between the far left and moderate wings of the party.
Polls came out today that nearly 60% of dems think HRC should be VP, 37% against. That 37% better not raise a stink if he does offer it to her, the majority believe she more than deserves it.
With HRC and BO on the ticket, the dems get 100% of their base.
I don't think it would be smart of him to minimize the historic nature HRC becoming first female VP (she after all will have gotten 50% of the dem votes for all intensive purposes) and slap half the party in the face by offering it to a balding-moderate leaning-gun toting-white man who wears flag pins.
We already have one of those in the race: John McCain.
I doubt many BO supporters are NOT going to vote for him because she's on the ticket. Most of her supporters (hopefully) will begrudgingly be satisfied that she will add strength and experience where he needs it and she'll be in a strong position to run again in 8 years.
If Obama really is about "uniting" he will offer it to her and tell his hardcore followers to deal with it. If he doesn't, there will be a lot more Independents come November - including myself.