Just from very unscientific anecdotal evidence from people posting on this board, etc. I've come to some (again, very unscientific) conclusions. Just because of what I'm saying below doesn't mean there aren't rampant counterexamples -- this is just how the odds seem to break, which of course could be different in any particular case.
1) Upgrades are more common at deluxes.
2) Upgrades are more common if you're booked for the basic category. (E.g., you're more likely to upgraded from garden to lagoon at the GF, then from lagoon to concierge; or from standard to woods at WL than from woods to courtyard.)
3) Upgrades are more likely to happen before you check in -- that is, by the time you get there, your room has already been upgraded as part of the room assignment process.
4) If you ask for an upgrade at check in, you're more likely to get one the shorter your stay and the earlier you check in. It is hard to say whether asking has any effect -- just because you ask and receive an upgrade, doesn't mean it was the asking that did it. You might have already been upgraded by the room assignor/controller 3 days earlier. My personal hunch is that if upgrades are available at the time of check in, you stand a far better chance by not asking and instead just being really really nice to the CM. But maybe that's just because this is my style and it's worked for us a couple of times.
5) The fewer requests you have made for your room, the better your chance of getting upgraded. This is kind of a counterintuitive one -- I think the point is that if you have specific requests, they will try to accomodate them first. The chance of an upgraded room also meeting your requests is unlikely.
6) The more buildings, or sections within a building, the better chance you have. This is a complicated one. Basically, an undisclosed factor for whether you get an upgrade has to do with the housekeeping. To the extent that the resort can keep people all checking out on the same day in roughly the same wing or building or floor, the more efficiently they can clean the rooms on that check out day. There's really nothing you can do on this one, but it's interesting to know.
Again, this is all totally non scientific, and could all be 100 percent wrong. Just my observations from personal experience, reading these boards, and talking to some CMs. I've posted some of this same stuff before and others have disagreed vehamently with some of it, so YMMV, buyer beware, and whatever other caveats you can think of! ;0)
It never hurts to ask.
Here's a bump up, anyway....maybe somebody with a lucky experience will reply.
If you are charming and polite and friendly with the CM checking you in, I really don't think asking is going to automatically put you on the black list! My parents travel extensively worldwide and they have no problem asking for what they want and actually they do get it! They are very classy and personable people and that shows and works for them. I am not bold enough to do as they do, so I just book my concierge room, but I think it is kind of silly not to ask if it is something you feel comfortable doing or to think if you do ask, you will be snubbed. 
