Preview at WWDC likely in June, followed by September launch.
Apple Sends Out Refund Information to Australian iPad Customers
Apple explains in the email that the new iPad is not compatible with Australian LTE or WiMAX networks, but does support "fast cellular networks" such as HSPA, HSPA+, and DC-HSDPA
The offer for returns is a response to complaints by Australian regulators that Apple's use of the term "4G" in its Australian marketing may have misled some customers into thinking that the new iPad would work on Australian LTE/WiMAX networks.
Apple is allowing Australians who purchased the new iPad before March 28th to return it for a full refund.