Your probably correct, there's seldom just one reason for such a move. Still HP is by far the biggest Itanium supporter, and everything I've read seems to indicate that they have the best Itanium infrastructure support. I guess we'll have to see how the tier 2 guys do.
I can see "big Iron " users going to IBM, but have a harder time seeing a move to SUN. I'm not sure I fully understand the situation yet.
As far as IBM goes, fat chance. Anything but token support from IBM for Itanium is really hard to imagine. IBM's support for Opteron has only been luke-warm, as far as I can tell, and Opteron doesn't directly compete with IBM's products. IBM seems to be fighting for the Itaniums market.
Do you really think Dell has the technical capacity to support Itanium? Xeon seems much more like a Dell product than Itanium. Actually, Opteron seems to be the best product for DELL, but that's not going to happen any time soon.