I guess my impression is that the capacitor for the Xenon poses a special problem with space constraints, because it is cylindrical and therefore thick and that is hard to fit in a thin cell phone. Whereas the sensors are flat and need to get larger in a two dimensional flat sense, so it seems a little more hopeful that they might get crammed into a thin cell phone. Indeed with the N86 Nokia at least claims they have done just this.
I also think that, with small point and shoot stand alone cameras that have better sensors, what takes up most of the extra room is the retractable lenses with optical zoom and often pretty large battery packs. To me getting in a larger lens is the biggest prohibitive factor.
I imagine though that as the electronics for everything else shrink (which they do continue to do), there may be room for a larger lens. And even the triumphant return of Xenon (for those who care about it). Also when you consider the size of the lens in even the smallest point and shoot cameras, compared to the incredibly tiny lenses in cell phones, there's a lot of room for improvement with cell phones, without having to approach the relatively large lens size in these point and shoots.
3mp vs 8mp.