When did the word "so" begin to be used to start a sentence?

In the last few years, I've noticed a growing usage of the word "so" to begin a sentence, especially in the context of higher education.

For example:

Interviewer: "What is the nature of your research"

Researcher: "So, what we wanted to find out is..."

It seems to be a replacement the word "well", or, more informally, "ok". Has this usage of the word been around for a long time and I'm just now noticing it? Do you think that is a valid use of the word?

Update: More than a decade on, the links in my answer are no longer valid. See other answers for a wealth of related references.

This isn't exactly an answer to "when," but the example that you provide--of a researcher--follows the thesis of this article on the phenomenon: http://seedmagazine.com/content/article/so/ (broken)

This article is linked from http://www.cbc.ca/quirks/episode/2009/03/07/horsey-aeology-binary-black-holes-tracking-red-tides-fish-re-evolution-walk-like-a-man-fact-or-ficti/ (broken) from the CBC Radio program, Quirks & Quarks (see the very bottom of the page, where you can listen to the show excerpt about the use of the word "so").

This usage seems like a discourse marker, a way of saying "right then, pay attention, I'm about to give you the answer". Seamus Heaney, in his fantastic translation of Beowulf, uses it so:

Conventional renderings of hwæt, the first word of the poem, tend towards the archaic literary, with ‘lo’, ‘hark’, ‘behold’, ‘attend’ and – more colloquially – ‘listen’ being some of the solutions offered previously. But in Hiberno-English Scullion-speak, the particle ‘so’ came naturally to the rescue, because in that idiom ‘so’ operates as an expression that obliterates all previous discourse and narrative, and at the same time functions as an exclamation calling for immediate attention. So, ‘so’ it was:

So. The Spear-Danes in days gone by and the kings who ruled them had courage and greatness. We have heard of those princes’ heroic campaigns.

To my British ears it doesn't seem new at all.

How a Man May Choose a Good Wife From a Bad (1602): 'So, let me see: my apron.'

I first noticed it a few years ago from Microsoft people giving sales / technical presentations. Maybe it started there?

I find it very annoying, and hope it will die out like some of the silly things we used to say when I was in college.