Some time ago I read a post somewhere, I don't remember where, in which the author predicted that there'd be fewer instances in Cataclysm, but that they'd be more polished than ever before. I don't remember any more details of this post - apologies, anonymous author - only that I disagreed with the notion.
I think with the way the dungeon finder has changed instancing, it would only be logical for Blizzard to make more instances than ever in Cataclysm. I mean, WOTLK started with twelve new dungeons (now up to sixteen), and at the time that felt like plenty, but now that people are running them over and over day after day due to how easy it is to get a group and due to the improved rewards, even that almost feels like too little. If you run heroics with any kind of regularity, it's become impossible to not get that feeling of "oh no, not this one again" with every other loading screen. So if Blizzard wants to keep people running heroics as part of Cataclysm's endgame too, they'll have to offer as much variety as possible to keep things fresh that little bit longer. The addition of new five-mans in patches might also become more common.
In the same vein, these new Cataclysm instances are likely to be shorter than ever to make chain-running them as painless as possible, and possibly less rich in story and lore, since that only tends to get in the way of repeated runs (just think of how many people hate Culling of Stratholme now, for having "too many unnecessary roleplaying elements" which slow things down).
With the dungeon finder granting instant access and teleports to places all over the world, it's also to be expected that fewer people will know what any given dungeon's place is in the bigger picture. In WOTLK all the instances were still reasonably connected to their environment: just think of things like all the quests in Coldarra sending you to the Nexus, or the Grizzly Hills storyline that wants you to take over Drak'tharon Keep. People were generally likely to pick up at least a few of those as they were questing in the nearby area, but in Cataclysm many will likely hop into the dungeon queue as soon as they start levelling, maybe never doing much questing at all. I could see Blizzard supporting this by putting more quest givers directly inside the instances so people can get a feel for what they are about without having to rely on outside quests to clue them in.
Lastly, I also expect them to add some new low-level instances. I don't know if it's true, but I read somewhere that the gnomes are actually going to take back Gnomeregan, which would mean one less levelling instance to go to, and it's entirely possible that other old world instances might suffer a similar fate with the way the world is going to be shaken up. These would need replacing of course, but even if we keep all the low-level dungeons as they are, the dungeon finder has promoted a new levelling style that's entirely limited to instances, and it has shown that there are certain "gaps" where you can do nothing but run the same instance over and over. The graveyard section of the Scarlet Monestary is a prime offender of this for example, and it would make a lot of sense to introduce something new in that level range to give people alternative dungeons to run, if the devs are trying to make the levelling process more fun anyway.
What do you think will happen with instances in Cataclysm?