Hey, I've also been thinking about similar needs, as discussed at:
http://forums.storagereview.net/index.p ... opic=20422 (which also links back to another thread on this forum).
Anyways, I like my current Hitachi 60GB/4200rpm
1.8" HDD in a modular USB enclosure (3.2" x 2.8" x 0.4"), which can
always run off USB bus power from just 1 port since startup peak power draw is just 2.1W. It's just that I want more capacity (FYI Hitachi will have 80GB 1.8" model soon).
I can get more capacity with
2.5" HDDs, but their startup peak power draw seem to necessitate always using 2 USB ports

If I can live with that, I was going to use one of the following 2.5" drives in a modular USB enclosure (4.5" x 2.6" x 0.4"):
- Hitachi's 120GB/4200rpm model (slower; startup power draw is only 4.5W)
- Seagate's 120GB/5400rpm model or wait for their upcoming 160GB/5400rpm model (faster; startup power draw is 5W or exactly the bus power of 2 USB ports)
Hitachi's 5400rpm models max out at only 100GB. As for Hitachi's and Seagate's 7200rpm 2.5" drives, their startup power draw is 5.5W which I'm worried is more than many computers can supply even from 2 USB ports.
P.S. What's ironic is that other than startup, the actual running operations of the 2.5" HDDs (seek/read/write/idle/standby) don't generally exceed the bus power of just 1 USB port!