Here's what I've gotten thus far
I asked the same question not too long ago, I've had my 15" for 2 weeks now.
1) Mouse. I have a small Kensington USB mouse, but I honestly find myself not using it and just using the trackpad.
2) Extra power adaptor. Keep on in my travel bag, keep one at home. Has been very useful to have.
3) I don't have a USB keychain, but what I do have is one of those CF readers. I used the CF card that came with my camera to transfer files between my two Macs, before I got my airport network going.
4) Good bag. I'm waiting for Waterfield to start making the Racer-X again. In the meantime, I have a Marware Sportsuit which serves just fine.
5) I have a Radtech screen protector and a set of Radtech Wildeepz. The Wildeepz are little pads that keep your screen from even touching the keyboard or wrist area. The screen protector (which I don't *really* need, but it also doubles as a screen wipe) is also very nice. Either way, I'd get some sort of screen protector (something better than the Marware keyboard pad, IMHO) quickly.
That's my list thus far. I'm looking into getting some sort of CoolPad thing at some point (may just make one myself).
Enjoy the PowerBook!
...Jim