AlyssaS - 11:42 pm on Dec 1, 2010 (gmt 0)
I've had one instance where tweets helped rank a page of mine.
Basically I'd recommended a product in conjunction with something else, and the manufacturer of that product picked up my page (because they had a Google alert set up?) and tweeted about it.
The page suddenly surged into the top ten after the tweet, then came and went from the the bottom of the first page for a few months, and then moved into the top three.
My guess as to what happened is that G knew that the tweeter was an authority because their twitter account was linked to from their website (so their authority wasn't just based on twitter followers, but based on reference from their website which was trusted).
My page then went on "trial" and I presume they moved it up to the top three because of user experience (people weren't backspacing back into the SERPs).
It's lovely when this happens, but very rare. I don't see how you could game authority like this, though you could attract attention by making yourself more visible.