I've seen a few devices of this size (the Archos 7 comes to mind; and I've seen them on the internet, not in person, mind you) and I think it's interesting to watch the industry try to figure out the correct form factor for this new niche that is emerging. Obviously it is going to be something larger than a cellphone and smaller than a laptop -- but what, exactly?
This Dell Streak, I think, is the exact wrong size. It's quite a bit larger than a phone, and it doesn't look like something that you want to carry around all day in your pant pocket. One of the reason cellphones have become so popular is because they are so small (and light-weight). Remember that for several years, the major thing about cellphones is that they were getting smaller and smaller? Compare a phone from, say, 2000, with a phone from today. Why would anyone want to reverse direction on that? It's too large for a phone.
On the opposite end, it looks too small to do any actual work. A netbook-sized screen is good for emails and browsing, but it's not very useful for doing serious business. And this thing is much smaller than a netbook. I don't think that's the aim, of course -- I think it's more aimed to the niche that the iPod targets; gaming, "always-on" style internet access, etc.. But I have to wonder if the device is too small for these things, as well. I think it might very well be.
But the overwhelming thing we seem to be seeing is that there are plethora of devices being released, each being in some significant way different from the next; companies are trying to find out what consumers want in a device like this. Maybe Apple has proven it with the iPad, given its popularity; they did that with the iPod, and now the market is full of MP3 players which are essentially iPod clones. But remember when MP3 players were first coming to market, there were many different form factors, many different storage devices (Sony had that thing with the mini CDs, for example), until it became clear what consumers want. The same thing should/will happen here; and I believe it's quite possible it's already happened with the iPad, and anyone making anything substantially different will wind up falling behind.