First iPhone 3g charge?

Probably a good idea to charge it completely at first. When completely charged, there will be a plug icon shown on the battery level indicator in the menu bar.

Some people like to "season" or calibrate their battery when new. If you are interested in doing this after your iPhone is fully charged, use your iPhone to the battery runs completely down. When it runs completely down, the iPhone will shut off automatically.

Then charge your iPhone's battery completely before disconnecting it from your computer or the wall charger. You can use your iPhone while it is charging and when completely charged and remains connected to your computer or a wall outlet.

I've been a little confused about the charging procedure. The manual didn't have much info. On apple.com/batteries I saw the following text "hat’s about two hours of charge time to power an iPod to 80% capacity, then another two hours to fully charge it", but that was mentioned in relation to the ipod. I've found that I get the "plug icon" after about 2 hours. Is this really fully charged, or should I leave it in for another 2 hours? In what way does the iphone battery differ from the ipod batteries?

When you see the plug icon, the battery is fully charged. You don't need to leave it for another 2hrs. The 80%/20% charging process is generally how a lithium ion battery is charged. It's not specifically just for only iPod/iphones, but any device that uses a lithium ion battery. The charging process is controlled by microchip.