http://www.thinkgeek.com/gadgets/cellphone/e8f5/
Yeah I've seen these, in fact I've used a Wii controller with my Android tablet to play with MAME. It's a very awkward experience to try and hold both a tablet and your controller at the same time. Only useful if you can set your tablet down on a table and sit in front of it, a position I am absolutely never in when mobile gaming.
The stick on joystick is an interesting idea but I can imagine all sorts of problems with that. For one, it's not a d-pad, it sticks out way too much, so you'd have to take it off and put it in another pocket or whatever when not using it. I don't see why they couldn't make a d-pad version though. I am sure every game has the virtual d-pad in a different spot, so you'd also need to be moving it around each time you play a different game.
I think unless Apple heavily commits to gaming I'm not sure d-pads and a button built into the hardware will take off any time soon. Oddly, if Apple did make it a spec to include a d-pad, I bet you'd see every Android device follow suit immediately for fear of not keeping up. It's a bit of a sad world that way right now.
I'm still trying to download Battle Squadron right now, unfortunately Apple's app store rarely wants to download or update apps, they just sit there waiting to download and never do until the moon is magically aligned with some planet. Don't get me started on how much I loathe Apple. :furious: