It's been a while, and I don't have my 3G iPad anymore (gave it to Mom), so if I make a mistake in the descriptions, you'll have to figure it out.
First make sure you've got 3G on, and are connected via 3G. Turn off your Wi-Fi if necessary so that you get the 3G indicator instead of the fan shaped signal indicator (if you see that you are using Wi-Fi. Other's have said you can do this through Wi-Fi, but I never had any success.
Now go to your 3G Data plan in Settings. I can't remember the right one, but one of the two options that mention 3G or cellular data only show up when 3G is on. That should be the one. It should offer you the ability to sign into your account. Once in your account there should be an option to change your password. You may have to dig around a bit to find it.
Best I can do without the 3G iPad here to play with. Mom's on the road with it today. She seems to rather like the iPad, which would makes it the fist Apple product I've given her that she actually uses.