Kotaku reports a major change in Final Fantasy XI; Players will be allowed to group with low level friends, with their gear and power level automatically scaling downward to the level in question. Previously, you had to actually have a job (FFXI's classes, you have one character who has levels in each job you have unlocked, and can switch at your player's house) that was at the same level as your friend's character, or neither of you could expect much in the way of exp. The new system means that you can literally level up with your friends without worrying about what level your characters actually are, by setting the party level at the level of the lowest character.
Some of the many solutions
FFXI is one of few MMORPG's that still has a very steep exp curve AND mandatory grouping. As a result, they really needed a system with this to help players get into the game with their friends. That said, Square is not alone in tackling this question. Blizzard has made (and/or will be making, whenever the expansion hits) a large number of assorted changes to the speed of leveling in WoW. COH has a system they call side-kicking that is kind of like FFXI's method but with significantly less in the way of rewards for the higher level player. I don't know much about EQ2, but I'm told that they actually allow you to solo to the level cap (a pretty major departure from EQ1). Warhammer is implementing a scaling system that will bump lower level players up to the appropriate level for given scenarios, though it currently won't bump high level players down (indeed, they're prohibited outright from joining scenarios they outlevel).
I think Penny Arcade summed up the situation best in a comic from shortly after WoW's launch. MMORPG's are nominally social games, and it falls to the developer to find some way to help that happen.
The Perils of Accelerated Gameplay
In FFXI, mobs award experience based on what their level is relative to yours, completely independently of what level the mob actually is. If one imagines that a level 70 mob is more difficult for level 70 characters to kill than a level 20 mob is for level 20 characters to kill, one could imagine an issue here; it may actually be EASIER to level by tagging along with lowbies. This is probably NOT what Square Enix had in mind, though their entire game system is very very dependent on convincing higher level players to help newbies through low level content.
Likewise, MMORPG's are largely a question of time investment to reward ratios. Tipping the scales in favor of new players/new alts can discourage existing players from trying new things, in the hopes that they will be nerfed later. For instance, I'm debating whether it's even worth bothering to level my Fury Warrior towards level 70 before patch 3.0 comes out, given that the patch offers less exp required per level AND the new talents, such as the impressive sounding Titan's Grip. (Note to self: If I am parking Greenraven at 61 or 62, I should probably think about picking up some 2H weapons NOW so I'll have them to use when the patch goes live.)
Meanwhile, Deathknights start off at level 55 with an epic ground mount, feats that any other character would have to earn over time spent playing pre-TBC content (which is generally the least polished stuff in the game at this point). This makes alts of other classes somewhat less appealing. I suspect that Blizzard WILL allow level 50-55 premades of other classes, but not until a later patch, say 3.3 or so, and they won't breathe a word about it until the last possible moment for fear of deterring people from playing alts now.
Where is the balance?
WoW's current system has its flaws; they're focusing on skipping players past lackluster old content and rewarding players for finding Blizzard more real-world money rather than on the actual gaming experience. (As others have commented, the new recruitment program would be a huge help in gaming with my wife if I can ever talk her back to the keyboard, but we're not eligible because she is a former subscriber rather than a new one.) The ultimate goal is for players to WANT to spend the time at levels as they come. Unfortunately, that requires spending limited dev time on old content that existing players may have passed, never to complete again.
Then again, the increase in solo-friendly content in just about every game out there suggests that developers are acutely aware of this issue. Hopefully someone will come up with something that's interesting AND an actual solution, rather than yet another bandage.