iPod: Not really. Looks and behaves pretty much the same as far as I can tell.
Also, finally the ability to go fullscreen in Safari in landscape mode (not sure why it’s not there for portrait??), and the Passbook should prove useful for plane boarding passes, movie tickets, coupons, etc.
From what I’m reading here and what I have seen on youtube I see no compelling reason to upgrade. The iPhone 4 does not have Siri or turn by turn directions, the map program seem like a downgrade with loss of street views, which I do occasionally find very useful, and just offers a slightly different ‘skin’ on some apps, which is nice but not valuable in itself. The do not disturb feature doesn’t do anything for me either, though is a good idea to have.
So unless something better comes along or iOS5 loses functionality I think I will just stick with that.