Actually the DS homebrew scene was alive and well last I looked, I would be surprised if there aren't aps (or, for that matter, the Ninty Opera cart) that could help you find a cab or use Facebook on-the-go (though why you'd want to use it at all remains a mystery). A DS/DSi with an R4 (or equiv) opens up a lot more options then you seem to think.

That aside, a PSP can do most of what iPhone does -- including make phone calls (Skype) -- right out of the box. You need a Wifi connect to do much of that, of course, which limits the utility of the phone part, but I dare say the games are WAY better then the iPhone. Beyond that, if you have anything but a 3000 (and with the recent release of the tiff exploit and ChickHEN even that's mattering less these days) there's a world of homebrew to explore. As is the norm with such things, some of it is amazingly good (better browsers, for starters) and some of it... Less so.