As the previous poster stated you can't use a USB mobile broadband dongle directly with the iPad but you could plug it into a Mac and create wifi hotspot using internet sharing.
This would be no good for travelling of course but there is, at least here in the UK, something called MIFI - this combines a mobile broadband dongle with a wifi hub and is about the size of a small mobile phone. This works out, for me at least, a lot cheaper than the 3G iPad and it can be used with upto 5 devices at the same time so I can have my iPod and 3 other Macs/ PCs attached as well if I want.
When out and about I just switch it on and pop it in my pocket.
Message was edited by: Dynamicworm