If a four digit pin is good enough for the ATM it should be sufficient for an iTunes purchase on my phone, putting in passwords sucks
Now, I really believe in having strong passwords that are different for the various services I use. To do this, I employ the use of 1Password on my Mac, iPhone, iPad, and my Windows 7 virtual machine. And it all stays in sync via the wonderful Dropbox. Using this, not even I know the various passwords for the different sites and services I use. But 1Password does, and I know how to get at that information.
However, I am forced to either remember my password for iTunes/App Store purchases/updates on my iPhone and iPad, or delve into 1Password each and every time I need to do anything.
I can see why Kevin seeks a PIN for iOS. And I am nearly inclined to agree. Apple provides the free Find My iPhone service, which gives an extra security to lock someone out of your data or erase that data entirely if your iDevice falls into the wrong hands. I think for most people a PIN would be sufficient. Much like an ATM, you need to have two things: something you have (the card) and something you know (the PIN). The situation is very similar with Kevin's idea. You have your iPhone and you know your PIN. And just like if you lost your card and report it missing, you can lock down or erase your iPhone if necessary.
I think the PIN system could work. It should at least be an option for those willing to take a little greater risk.