Probably not the ones you build yourself. Some have varying current requirements and some won't power up with some drives with every USB socket (i.e. they require more current when the HDD is spinning up than USB sockets are 'legally' allowed to give out. A problem in certain cases).
I would recommend the Freecom HDD's. Bear in mind I use USB drives for PC/Mac use - don't really have a use for Firewire drives, which as the post below says will allow you to boot from it.
The FHD-2 is an aluminium encased drive with a minUSB socket. Pretty low profile, smart looking and very cool in use. 4 still used at home, about 10 still in use elsewhere, 1 failure in just under 2 years. Very reliably bus-powered. Practically every USB port I've plugged this drive into powers it, which is not the case for all USB 2.5" drives - especially the third-party enclosures I mentioned above. I'd recommend this if you don't feel you need the drop-resistant features of the Toughdrive mentioned below. Another feature is that if you dismount the drive, it switches off (can't remember whether it worked all the time on a Mac). The data sync button doesn't work on a Mac.
http://www.freecom.com/ecproduct_detail.asp?ID=3542&CatID=8020&sCatID=1146438&ssCatID=1146441
I was casting around for replacements to the FHD-2's and having bought a few, I decided on the Toughdrive as the most viable replacement for actually portable use.
The Toughdrive is of course bus-powered, and is just like the FHD-2 in terms of powering reliability. It has a captive USB plug (and an extension cord), rubber buffers around the drive in a plastic shell, which is itself encased in rubber. Gets warmer than the FHD-2 in use, but extended run-time testing has found no functional issues. I initially bought a batch for my own use, found it excellent and we acquired a large number recently. No failures to date but it's fairly early days. The captive, flexible USB plug is a good idea once again for actually portable use (Forgot cables? you'd be stuffed with a regular drive. Not with the Toughdrive), but the elastomer it is made out of will break down and become brittle after about 5 years of use.
http://www.freecom.com/ecproduct_detail.asp?ID=3506&CatID=400
All Freecoms seem to use Samsung drives.