On the contrary, the tech support rep who misinformed you was not correct. As I write this, my sleeping iPad's battery is charging from one of the USB 2.0 ports on my late-2008 unibody Macbook Pro. Yes, its charge level really is increasing — I've verified that by waking it briefly every few minutes to check the charge level indicator. You may not be able to charge an iPad while you use it, but you can indeed charge it when it's shut down or sleeping, provided that your MBP is awake and no other Apple device requiring more than 500mA is connected to its other USB port(s). Read this article for more info:
FYI, while this is going on my MBP's second USB port is powering the USB hub in my 27" LED Cinema Display. But the two devices plugged into that hub both have their own AC adapters and are currently turned off anyway, so the hub isn't pulling any significant power from the computer.
Since I began writing this post, my iPad's charge level has risen from 71% to 76%.