Last question first:
Dell has just recently started using AMD. In their servers. Why now, I have no idea. Speculation says that Dell was getting cheaper prices from Intel for going exclusive. Though technically, now that Dell owns Alienware, they can claim that they use AMD...
Apple? Apple's just weird.
But anyway, Apples used to use IBM processors, and then they started using Intels. Speed issues or something. I don't know why they chose Intel over AMD, but I believe that they aren't easily switchable is because Intel built those processors specifically for Apple, with strange tech that nobody else uses.
Btw, AMD had price drops in mid-May, I believe.http://www.dailytech.com/article.aspx?newsid=2041
This may affect upgradeability, at least in the long term, but AMD is just now switching to socket AM2. They're selling for roughly the same amount as 939, and are already possible to find. Unfortunately, as of right now, it offers almost no performance increase.
I would assume that AMD is going to drop prices further, or they'll get slaughtered by Conroe.
Also, it is possible to get processors from AMD for under a $100, but they have to be socket 754 (budget) or AM2. I coulda sworn they had Semprons on 939, but I didn't see any when I just looked.