On a new domain I'm there today and wasn't last night. In fact, it's the first time I've seen it in Google. First crawl since the site went live by googlebot shows only three days ago. (Not that I'm complaining - but it strikes me as strange.)
Yeah, I've never had a "fresh" tag before, but I got one for about a dozen or so pages yesterday. Those get a little bump in the SERPS which, obviously effect everything that's gotten bumped. Kinda cool if you're the one with the fresh tag, kinda sucks if you're the one wondering why you just moved down 10 slots.
I have a few pages which show up with fresh tags, but the site has no PR - I mean grey bar. I wasn't spidered until the 26th of August. Surley if the site is in the index it should have some kind of PR?
It is a bit weird. I thought the dance is over but now suddenly a very old page of mine that I just changed a bit and that I hadn't updated for months suddenly shows up in the top 10 for an important keyword. It also has the date tag. I didn't know that old sites can change their positions between dances that much.
As mentioned, the "fresh" tag means it has a date in the results. Once you've gotten that tag, Google tends to come back every day or and check those fresh pages for new content. If it has been updated, the new content goes right into the index. If it hasn't been updated since the last visit, the fresh tag usually goes away (and, unfortunately, so does the newly indexed content - it'll go back to what was there at the last dance.)
Fresh tags move in and out constantly, and I suspect that is most of what we see in the fluctuations from day to day between major updates.