I think WiFi has MANY great potential uses in an iPod.
1. syncing -- only sort of useful.. would be cool for minor updates but not a big, first-time big sync. Then again, it would need a separate power charger.
2. AirTunes remote.
Those are the obvious ones... but people, think BIG.
Take your iPod to any WiFi hotspot, and maybe get some plain-text email in a basic email reader application (which you set up on your Mac, and have the settings sync over).
In the future, how about iTMS2Go. You want to buy a track on the spot, when you're not near your computer (say, you're at a party)... Maybe some version of iTMS with the iPod navigation system. It would sync right back up to iTunes. Let's see some other player/music store do that!
Rendezvous iPod music sharing. Hey, some other 4G iPod user is on my train, now I can stream his non-protected music... and if I like it, click a button and go to iTMS2Go and purchase it myself!
I know some of this might seem silly, maybe it is. Just brainstorming -- imagine what talented professionals could come up with!
Of course, we won't see WiFi in an iPod until battery technology is better.