So... for archive purposes:
I decided to buy a 160GB 2.5" Seagate solution with FW800 and USB2 from OWC:
http://eshop.macsales.com/shop/firewire/on-the-go
It arrived last night and, as hoped, worked straight out of the box over FW800 without either the drive OR the laptop plugged in - I'm very happy
To test transfer, I backed up my home folder (about 43GB, with 19,000 files in it). It took just over half an hour - not as fast as I'd have liked - but still pretty speedy. At some points it was copying in the mid 30MB/sec range, but mostly around 20-25MB/sec (according to Activity Monitor).
Attempting the same feat over USB2, required AC power to the HD, and copying was around 12-14MB/sec. (The first 10GB took about 15minutes, then I got bored).
The drive has no obvious vents and is made of clear polycarbonate (I think), is actually a little quieter than the 100GB 7200rpm HD in my Powerbook. It got slightly warm, but nothing concerning at all. The carry pouch is kind of cheap feeling and a bit oversized, but it fits in the FW800 and USB2 cables to become snug - but no room for Power adapter.
This pouch will be perfect protect the HD during storage/transport, since i don't have a dedicated computer desk/area.
If anyone is interested, Weibetech makes a bus-powered 3.5" 7200rpm drive (
http://www.wiebetech.com/products/ultragbplus.php) - but I chose against it because of size/weight, and the possibility that the powerbook would not provide enough juice.