Since I'm doing a variety of network programs for iPhones and iPods (FTP OnTheGo, ContactClone) I've got an iPod touch for testing, as well as my iPhone 3G and the older iPhone that's now my wife's. With the network programs, I've been changing settings and noticed a difference in the number of wireless networks they showed.
I did a test sitting at my desk...
The iPhone and iPhone 3G saw 2 wireless networks in the settings, sometimes a third would appear for a few seconds, but would disappear again.
Exactly the same with a first generation iPod touch consistently saw 4 to 5, and a few times as many as 8 wireless networks! The quality was low, so I'd have trouble actually connecting, but was a very surprising difference. I guess the antenna in the iPod is a lot better than the one in the iPhone.
I'm curious how an iPod touch 2nd generation would compare...but not curious enough to buy one yet :)