Well, out of the total 100 watts, the memory controller(s) probably does explain at least a material percentage of it. After all it's more than increased addressability they were going for, but similar bandwidth levels as well; on DDR2, that's going to require a lot more relative to the XDR variant of the chip.
The wire tracing on the motherboard is going to be ugly, that's for sure.
The package must also be really huge. I wonder if they were so pad limited because of that DDR2 controller that they needed to put some dead space in the layout to make room. It's been done before, but people usually go through revisions to improve the layout. That could theoretically account for some die space issues, as could routing signals through the chip (assuming they had to make changes here to account for the wider bus), which could have been saved by adding more interconnect layers. In any case, I'm grasping at straws myself because both power and die size had little improvement at this shrink.
As much as an XDR controller is low power compared to DDR2, I can't see that alone accounting for a lot. Leakage increase is a definite possibility (especially since it's on a strained SOI process (?) -- someone else may be better suited to comment on this), and I'm guessing this was probably a bit rushed because they may have a lot of potential customers eyeing Cell whose appetites they wanted to whet ASAP. Normally, I'd expect IBM of all companies to do better. Sure, these may be TDPs rather than a true power consumption, but you'd still expect that to go down a good amount with a process switch.
Well, we can only hope that a PS3 counterpart will surely have more work done to it to bring power down, as the absence of a single working SPE will hardly account for more than a few watts. Late in the development of the PS3 Cell, we were hearing all these things about Sony and Transmeta doing some work on power-saving techniques, and it was too late for the initial PS3 Cell production lines, but maybe not for 65nm. I honestly don't know what they could do or what they're not yet doing, but it would take some serious complacency to do much worse than the current state suggests.