One of the new features of the iPad 3 is the ability to utilize high-speed LTE networks and share the connection with up to 5 other devices. As of today it doesn't look like AT&T will be ready to support the iPad's hotspot feature before its March 16 launch. However, Verizon will offer free hotspot support for the 4G models.
When pre-ordering the new iPad you must choose what carrier you would like to use. The model you select is configured to work with either Verizon or AT&T, not both! So you'll have to decide which carrier's 4G LTE network you prefer. You do not need to activate your service right away, and you can choose a monthly data plan at a later date when you need it.