The move from Tiger to Leopard will be, as usual, seamless. Just stick in the installer and go.
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As far as I understand it, the current OS is already Universal Binary in that it will run on Intel machines or PPC machines. I could be wrong on that though, there could be a dedicated Intel processor OS.
If it is not already UB, then Leopard will certainly be UB. It is going to run on both PPC and Intel processors.
Either way, UB is not going to make any difference to the actual running of the machine and just because Boot Camp will be part of the OS, should not make any difference. It could be that Boot Camp gets upgraded so that it no longer requires a partitioned drive.
If you run Parallels, you will not have to do anything to your HD as it does not require XP to be within its own partition.

Any answer to your last question would just be a guess but, if the rest of the OS installation is anything to go by, the answer is that the transition will also be seamless