Can't say I've ever heard of anyone running the Focusrite as a field preamp so based on that I'd guess the answer is it will not run without a computer interface. Stock UA-5 will not run as a standalone but the modded ones do. The digi mod you see mentioned is the way to make it standalone or one of the units modded by Oade or Busman which upgrades the sound quality too.
(digimod) Edirol UA5 won't quite run standalone.
It will convert go:
mics > ADC > digital (spdif, coax or optical)
mics > ADC > DAC > analog (line out)
So, you can get analog, but you get an extra ADC/DAC stage in there. It would work in a pinch (eg., for a patch or backup rig), but not really recommended.
There are lots of other boxes that *will* work as a (standalone) pre, though. Presonus firewire boxes do. I'm guessing their USB boxes do too, but I've not confirmed this.
If anyone gets the Focusrite, I'd love to see the *guts*. In particular, I always want to know what power supply voltages the opamps work at. Most common is something like +-5V (10V total), but some really low end ones (Tascam US-122) run at +5V only. Professional grade gear should run with +/-12V or +/-15V. Note that even a lowly Behringer mixer runs at +/-15V. They all run the same cheap opamps (eg., NJM 4158 or NJM 2068), but running at higher voltages is better.
Richard