Wow. It isn't even close. Apple blew by HP, Dell and even MagicJack (!) with the most visits to a website in May. According to the people who compile such statistics at Nielsen, Apple drew 55.7 million unique viewers, more than double what next ranked HP did. Much of that may have been driven by anticipation of the new iPhone. Here's the chart.
Also interesting is that Nielsen computes the average visitor stayed on the Apple site an average of 1 hour and 14 minutes. That's a lot of reading and watching demo videos.
Nielsen also took a look at blog mentions of the iPhone in early June, and the numbers are pretty striking there too. You can see the peak as the release date approached, with another peak on release day June 19.
They say if people are talking about you it's a good thing. By that measure things are really great at Apple.