They wont let you use the 1GB/$20 plan for the ipad.
I originally got the Galaxy Tab with that plan, I bought an ipad 2 and they forced me to switch to the 2GB/$30 plan.
But I'm sure if you keep on calling the right c/rep at the right time will let you keep the grandfathered plan.
For me I needed the extra GB so I didn't mind upgrading the plan.