It was discussed on this forum before. Can't remember off hand now where it isnow. But you can search my posts and find it (there are a lot of them to search through). Someone noticed this (I have too). When you charge your 3G using the wall, the 3G charge will last a lot longer. When you charge via your USB, it cannot provide enough power to fully charge the 3G. Even though the 3G shows 100%, you will find that it runs down a lot quicker than a wall charge. When the phone goes into standby, it is not being fully charged properly. A USB can only provide so much power, and it's not enough for the 3G, that why it's advisable to use a powered USB hub.
Weird I know, but I've experienced this myself. I now use a wall charger at work also.
It could be another urban legend. But I started noticing this and I no longer charge via my PC anymore. Try it, you will notice the difference.
Originally Posted by Coolstrategist
Ravman...where did you learn this? I probably use my laptop computer usb port 90% of the time to charge my iPhone and the charge is full every time in a reatively short period. Most of the time it is on standby.