Actually, in this particular example, if Dell's BIOS specifically prohibited Solaris from running properly, Sun would have to hack their Kernel to work around the BIOS issue, which I doubt they would be willing to do.
But at the same time, you are correct, Dell has no obligation to support Solaris.
The real answer is, if you want to run Solaris, you buy a different machine that specifies and advertises that it is Solaris compatible.
Apple is in a grey area here. They are the first manufacturer that I know of who is marketing their machine as Windows capable, with Boot Camp, but who say they do not support Windows. There is nothing wrong with that, really, just as long as they are willing to accept a higher return rate from Windows users who were willing to make the move over to the Apple platform.
If Boot Camp shipped in Leopard with this issue, then I would say that Apple is entirely responsible. Right now, considering Boot Camp is beta software, the responsibility lies with the end user.