4e tackles this in the combat context via the use of combat roles.
It still needs work, though, in relation to skill challenges. Does anyone have experience of running skill challenges in such a way that the spotlight expands in the way Janx is describing? What techniques did you use?
You've actually taken my point to an area I hadn't given much thought to.
I assumed, relatively speaking, that if the party was in a room with monsters, everybody was probably involved in the fight if they weren't dead or something.
I had been thinking about talking scenes, where the PC is pursuing his "personal goal", and could easily do so with out the other PCs, but has an opportunity to bring the PCs into it so they WOULD be there when a fight or other such scene came up.
For example, Bob wants to go check in with a contact who used to know his father, so he can learn how he died. The party just got into town from the dungeon. He could just say, I split off from the party to go do this. Or he could say, "hey Jack, I've got some personal business that I could use a man to watch my back."
Suddenly, there's another player in the scene, so now two, instead of one players are actively involved for a few minutes.
And Jack, could easily say, "Sure, I got yer back. But you know, if this busineess is dangerous, maybe you should tell us about it, and we can be even more prepared. heck Wyatt's got spells he could use to spot trouble."
Basically, the players, in the interests of always getting to do stuff, support other players in their personal goals, versus forking the party and having dead time.
Now for what Pemerton is implying, Skill Challenges to me strike me as "the best man for the job" gets to roll dice, while everybody else stands back, or at best "lends support" for the +2 bonus. I haven't seen 4e's definition of a skill challenge, but the concept seems to be "a situation that requires the use of a skill".
I would bet, that the way to incorporate more people, is to make a challenge have multiple steps, with different skills, and potentially, simultaneous skill usage in order to suceed. Basically Bob has to roll a Balance, while Jack does a Climb on top of him, to reach higher.
The other part to solve, is that a skill challenge doesn't get stuck on a simple die roll. Making a situation that has multiple diverse skill solutions means that Bob gets to try his way with his skills, then Jack gets to try if Bob fails (or perhaps they both try at about the same time, if there's no reason not to try both).