"Without all of the modern technology that we UL'ers rely on just how ultralight could we get? Is it even possible to be ultralight without a lot of modern-day, hi-tech equipment?"
Completely depends on your environment. I read a very interesting book about disease and geography, and a very interesting point was made that if not for technology (i.e. fire, clothing, agriculture), mankind could only live within a 1000-mile strip on either side of the equator.
Thinking about this in terms of backpacking, it made me realize that if one limited one's backpacking to tropical regions, one could probably get by with very little gear. The opposite extreme of course is living near the poles (complete dependence on clothing and hunting tools). Where you live in between these will dramatically effect what gear (aka technology) one needs to 'survive.'
And of course, the whole concept of 'survival' is also completely dependent on how long one is talking about surviving. I used to enjoy watching Survivorman; until I realized he's not truly surviving, but rather sustaining until rescued. Technically he doesn't need to forage or hunt, since it shouldn't kill him to not eat for a week. On the other hand, if he had to be out there for a few years, I think he'd quickly suffer malnourishment and starvation eating the very little amount he eats while on the show. But one would expect that, after the first year, he would have built a more permanent shelter, grown some crops, traded tools with locals, etc. So long-term survival would depend more and more on 'technology' to survive. When does this cease to be survival and become adapting? Be forewarned, I have not read or seen Into the Wild, but I think if the main character really wanted a chance at his endeavor, he should have been thinking more about adapting than surviving.
Perhaps surviving, then, could be defined as sustaining one's life by oneself, with no help from anyone but yourself. In this case, UL technology is certainly not needed. But frankly, the types of places many of us enjoy backpacking are places that 'survivalists' would not choose to dwell. Therefore I don't think the tools required would be really the same. True survival tools would be an axe for chopping wood to make a cabin, and seeds for planting next year's harvest, for example. Think about how impractical the Pacific Crest Trail is; what 'survivalist' would build and try to survive on a route that follows the highest, most difficult terrain to get from one point to the other? Most of the food suitable for human consumption would be down in the valleys. No, we walk that route purely for recreation, and to survive -- that is, survive until rescued by a SAR team, is dependent on the technology that allows us to journey on such unfit-for-sustaining-human-life routes.