I got a sudden spike in my regular daily traffic last week, and it's all from Google, and all for an article that I wrote quite a while ago.
After googling a bit, I discovered that Google apparently does a full rebuild of their indexes approximately once a month. That means that even if it knows about your site, chances are if you're a small site, it won't pick up new pages for a while.
That means keeping your URL's constant is really important. If you're like me and you switch blog systems way too often, then you're destroying your articles even before Google can get to them.