I can't imagine that the interview, or the BBC story to which Robinson was reacting, will change many people's minds. Robinson's personal style is pretty well known; he has been MP for East Belfast for over thirty years now. I would be rather astonished if there are a lot of potential DUP voters out there who watch it and are shocked into changing their vote because he gives a journalist a hard time. Of course, I haven't seen the interview and might change my mind if I do.
On the actual facts of the case, Robinson agrees that he bought a potentially valuable piece of land for £5, but it is also clear that he sold it for £5. The BBC case that he (or his developer friend) evaded tax in this particular instance is not very strong, even by the BBC's own account. Anyone with half an ear open is aware of the close links between property developers and electoral politics in certain areas of Northern Ireland, and there would need to be a more visibly smoking gun to make a real difference to Robinson's reputation (or the DUP's).
One piece I have been able to watch is the Guardian's video blog about North Down, where Lady Hermon is presented as a true Labour supporter who could hold the balance of power on her own in the next parliament. It's a rather smug Guardian piece; I winced at Harris's errors ("Republican" instead of "Nationalist", mispronouncing "Bangor", misspelling Ian Parsley's name) and a lot of it is him burbling to the camera or to the locals rather than actually getting their views. Ian Parsley has denied (scroll down to comment #4) the jibe that he didn't know what "legal highs" are, which is actually the most serious blow against him in the piece (other than his own account of himself).
Readers (if any are left) will have observed that I'm unduly fascinated by North Down, where the UUP, who won the seat in the Westminster elections in 2005, are not standing but backing the Conservative candidate, and the DUP, who got most votes in the Assembly election in 2007, are also not standing but backing the incumbent MP. This means that the parties which collectively got over 85% of the votes in 2005 will be absent from the fray this time. I know I'm unhealthily obsessed, but I still think this is unusual.