Ever try to send an e-mail with a telephone keypad? Even with predictive-typing assists (that guess as to where you're going with your hunting and multi-pecking) it's still painful, although high school kids (and Europeans with big cellular voice charges) seem to love it. So why not add a mini-keyboard to your phone? Some phones offer this option, but it's another thing to carry, lose, and drop. And that larger screen we desire for reasons of legibility or simply a little more screen context for Web-based information or reading e-mail means the mobile communicator isn't going to be small at all, and likely not very cheap. While there are some economies of scale, putting everything in one box almost always means a higher price and at least some compromise in functionality.
So I don't expect that we're going to see a popular single universal communicator any time soon. But there are lots of good approaches on the market today, including products from Handspring, Danger, Inc., and NTT DoCoMo (only in Japan for now). We're also seeing a number of great camera phones like Nokia's 3650, which can even record and distribute video in addition to still images, despite its questionable retro keypad layout. And manufacturers are now producing cell phones aimed at specific user constituencies, like Nokia's N-Gage (which is designed primarily for playing games! When all is said and done (and dialed), though, the choice is going to boil down to one of individual lifestyle and work style. You may need several mobile phones to meet all of your needs. A separate PDA will also likely be the path of choice for most people for a while longer.
As an aside, I think, like many of you, I'd be happy today if I could just get decent cellular voice service that didn't require me constantly having to ask "can you hear me now?" and that didn't drop calls so often. It doesn't matter who your carrier is; this industry still has issues - and they are getting old. Perhaps before we shoot for the ideal we might just focus on getting the basics right. But, regardless, I'm always going to want more.