If you search back through some of the older iPod Upgrade Rumour threads, you'll find that there is ABSOLUTELY NO EVIDENCE WHATSOEVER of any kind of upgrade to the iPod. Sure, there will no doubt be a HD upgrade (though why anyone would need more than 20GB is beyond me) but beyond that...
So go ahead and buy one. I was going to wait when I first read the rumours, and when I dug deeper and found them to be completely baseless, I went ahead and bought a 10GB iPod and have been loving it.
One thing, though - you'll want to buy a seperate case to put your iPod in - the case that comes with the device has an amazingly weak belt-clip - I've broken two of them in a period of three weeks...
Enjoy.