Yes, it is possible to upgrade the drives. Some hard drives like the 40 and 60 GB are thicker than the 15 and 30 GB ones, so a thicker iPod metal backing is necessary to accomodate the new drive. If you can find one that fits physically, the drive will work in your existing case. In the case of the 5G iPods, the 60 GB one is thicker than the 30, and the biggest capacity drive in the 30 "form factor" is the very same 30 GB. The next step up, 40 GB, is a taller drive.
I don't think you would see a noticeable decrease in battery life with increased capacity.
Not sure... I would gladly do the upgrade if you could find the parts. As for doing it yourself, it's not terribly hard, but I have seen people crack the LCD attempting to open the iPod. Doing this will void your warranty if Apple finds evidence of tampering.
From a 60 GB iPod, there shouldn't be any issues going to 80 GB when it's out. But again, the 30 GB iPod will need a thicker back to handle 40 to 80 GB drives.