I've been firmly in the "McCullum was right" camp since the beginning, but when people pointed out the similarity to the underarm incident, I initially struggled to come up with any compelling reason why the two are different.
However, now that I've had time to think it over, I can see that the only similarity is that both incidents were technically sanctioned by the laws of the game.
The differences are thus: while Muralitharan is largely, if not entirely to blame for his dismissal, Brian McKechnie and the NZ side played no role in the decision to bowl the underarm delivery, other than having scored all but seven of the required runs for victory up to that point.
Also, Murali's runout was a spur of the moment action on that part of McCullum, rather than a calculated, planned move, and at any rate, there's plenty of doubt as to whether McCullum even knew whether a run was being attempted or not.
In addition we have the comparative match situations. The result of the test was still very much undecided, and when you consider the players' professional obligation to do what they can to win, there really wasn't much option but to run Murali out. On the other hand, Trevor Chappell's underarm delivery was to a number 10 batsman who was just short of rubbish, and needed to hit a six just to tie the game - remember, New Zealand could not have won unless that last delivery had been a no-ball or wide.
In conclusion, I'll also register my bafflement over this so-called "spirit of cricket", when what is being spoken of is really no different than the general sportsmanship expected of any athlete, playing any sport.
One last point: I think a few Aussies are a bit ill-informed if they believe we kiwis are still upset about underarm. They'll tell sheep jokes, we'll hassle them about underarm