I tend to use whiteboards with a team who are all in the room with me, rather than as an individual (although I'm thinking of installing one in my home office, just to see how useful it would be). The iPad is less useful in this case, although I suppose you could get a big TV screen connected (or apple tv). Also, there are apps specifically designed to take and share photos of physical whiteboards…

The real killer for me is still the lag. Handwriting is just painful, although perhaps simple diagrams are not so bad? It seems to me that an Android tablet could distinguish itself by providing both very low lag with something like the Wacom accuracy. The iPad is unlikely to ever reach the ideal 1 ms lag -- I suspect the retina displays have relatively high response times (but have no evidence).