Yeah, I've only gotten a free upgrade a couple of times, but I'm Starwood Platinum and that may have something to do with it. I have had the experience of a free upgrade from standard to a suite (amazing suite actually) with a second bathroom, and overlooking the top of the fountain at the Dolphin for free, but usually we pay for the upgrade...my point though is that the upgrades made onsite are always very cheap. There was only 1 time I couldn't upgrade, and that was during the S&D F&W event and we had booked 4 rooms, they could upgrade 2 of them (we paid for it, again, like $20 -$30 bucks a night) but the other 2 had to stay standard. Its a gamble but its worth it, IMHO.
You should never book with the idea that you will definitely get upgraded. Since its just DH and I right now, we can be pretty fast and loose with planning. We are going in early Sept. with my SIL and BIL and my 5 year old nephew. We didn't even tell them about the possibility of an upgrade, but I think at that time of year, we'll get lucky (of course, we don't mind paying the nomimal fee).