... introducing higher-level monsters (i.e. a Pit Fiend, etc...) at lower levels? Or prevents higher/lower level characters/monsters from meeting and mingling, a la Planescape? I don't think any game mechanic could prevent it, if the DM chooses to do so! Unless WOTC intends to personally supervise each and every game session, it's impossible! So YES, some version of Planescape WILL be possible in the new Cosmology, whether WOTC likes it or not! The rules may state that characters of "x" level should adventure on the Material Plane and not the Astral, but the DM can just ignore it... Looking at the 4E Pit Fiend stats, I can't see what's to prevent a DM from throwing it at a 1st Level party if that's what the DM choses to do... all the rules for running it are there, so what's to prevent it?
How can WOTC possibly prevent a DM from...
Of course, WOTC can't really make me switch to 4E either. I for one have absolutly no problem butchering 4E for any rule changes / new monsters / new planes I like, then just stuffing them all into my Planescape-filled 3.5E.
But the whole "Will-You-Switch" debate has been going on for quite some time, so I won't get into all that here.
Sure that can't stop you from trying it but it's bound to be very difficult with the way it'll be designed.
One word: ninjas.
They cannot stop them at all.
And mind you I have met some sadistical DMs who like to throw high HD monstrosities at low level PC, worse still they use random encounters tables to justify that.
Seting high level monsters against low level PC: that stuff is old as game itself. It is not fair to PCs but if used properly it can build drama, introduce archvillan, give reason for PCs to hate him, etc...
But then again some people cahrge at such things...
Yeah, that "introduce enemy party can't possibly destroy, then watch him slap them around before skipping off to become their long term goal" thing drives me up a wall ... probably because it is so overdone.
When I run I have a rule which states no villain shall be encountered more than three times, and no villain can be behind more than one third the party's problems. (unless they are actively chasing said villain, then he'll do whatever he can to hinder them ... but at that point they've made a choice, rather than being saddled with an uber villain who won't stop taunting them and meddling in their lives.)
I almost never break those rules, because I hate reoccuring villains myself ...
Planescape is great because you can have the party encounter a high HD monster and still walk away, if they play their cards right ... far fewer encounters are about combat than in the more common settings. (not a response to anyone, just a comment)
I am still using a varient of 2E, and have weathered all the changes since with little difficulty, so I doubt 4E will become an obstacle for anyone who doesn't allow it to be. Use what you like, discard the rest ... I think that has been a gm montra for ... well, even longer than I've been playing ... and that is saying something.
I know what you mean: it either "reoccuring villain" or "monster of the week" dillema. Both are overused.
It is time for something new: "Reoccurimg monster villain of the week" prehaps... 
I think the worry is that monsters are being made too one-dimensional, rather than that the rules will forbid the inclusion of higher-level monsters in lower-level games.
From the previews, it looks like any abilities that can't be used in combat are being dubbed "unimportant" and being left to DM fiat. Which is fine, but if I want to include noncombat abilities, it would be nice if I didn't have to think of everything myself.
Yes, of course, but the planes seem a lot less flexible and accesible now. A 1st level character with the right papers and a bit of luck could survive in Dis in planescape, but the planes probably won't be as survivable (that is to say, big frackin' dungeons)