I use the older Nexto ND-2500 (currently with an 80GB drive, but can go up to 120GB). It has a CF slot, and is very fast with current cards. I carry a CFII-sized SD adapter when my wife's along, as her camera uses SD. Her UltraII SD cards read at a decent rate with this. The ND-2500 has FW400 and USB2 interfaces, and the external battery is 7.4V LiIon: I have a simple adapter so I can power it with BP-511 batteries if required (unfortunately for this the newer ND-2525 runs on 3.7V). It can be charged via 12V or via Firewire, so that's simple.
Currently I'm still travelling with a 12" PowerBook. I also travel with one or two external hard drives (still 60GB units, but I'll upgrade them in time). Both of them are FW400+USB2. And a USB2 CF/SD card reader...
I have the ND-2500 with me in the field, and when a card is filled it gets copied to that and then put back into my card-carrier flipped over (to indicate that it's full, but has been backed up once). I try to carry enough cards to get me through a whole day, but when shooting wildlife the giggage can mount up quickly. I have sometimes had to start re-using cards, and at that point at least I had one copy on disk of those images.
At the end of the day the rest of the images on cards get backed up, then the new files on the ND-2500 get synced over to one of the external drives so we have two copies of everything. We usually pack the backup drives in my travelling-companion's luggage (or at least put it in a different part of the vehicle if we're driving/boating).
If the Nexto died (or the CF/SD adapter died) I would still be able to back up cards via the laptop. If the laptop died it would be harder to make duplicate copies but at least I could use the Nexto, and when it filled up I could swap in one of the other drives (I normally have a screwdriver in the toolkit, and like most of these devices the Nexto just uses FAT32-formatted drives).
If I had to go on multi-day trips without a laptop I'd just go with two PSDs (at this point it would probably be the ND-2500 for in-field backups, and either an ND-2525 or something slower with a display screen for evening backups) and back everything up to both devices. But the laptop serves lots of other purposes (e.g. intermittent email, downloading GPS logs, sorting images with LR) so I prefer not to have to do that.
There are lots of ways of sorting something like this out, and unfortunately each decision you make will have a trade-off. FWIW, the above system worked well for me last year in Africa.