Well, the Dell Streak is 5".
I fully expect there to be more devices of such a size, including tablets that will double as cell phones - though probably not to be held up to your ear, more to be used via Bluetooth head pieces
I can't see going any bigger, personally. I don't like belt clips so my phone must be pocketable, and the Droid X is right at the upper limit of pocketability for me. I like the big screen but it's really pushing it. I think 4" might be the sweet spot, but the problems with the Samsung Galaxy S series kept me out of that screen size. I'm under no illusion that my needs are the majority's needs, though, and I think we're going to experience a fairly rapid convergence between cell phones, tablets and e-book readers as technologies get smaller and screen quality gets better.
I think the Samsung Tab is really aiming square at the heart of this trend. A quality tablet running a modern mobile OS that's far bigger than a normal cell phone but smaller than something like the iPad. For some people, they might be happy to carry it around, especially if they aren't the type of person who needs a cell phone attached to them at all times and can afford to leave it in the car or whatever when they don't want to carry it.
I think it'd be pretty interesting to have a tablet with a removable cell phone module that would let you detach the cell portion and essentially carry a dumb phone with you for times when you needed call functions but nothing else.