You can always cancel you online order unless the status changes to "prepared to ship".
I think people need to sit back and take a deep breathe, I have never been to a major Apple launch and not walked out with a product in my hand. Apple will have more than enough iPads for everyone this weekend and a press release ready to go Monday morning congratulating themselves on selling a Million over the weekend.
The March 11th release is only for the US, other countries have to wait 2 weeks, so Apple is prioritizing their shipment to make sure they have enough to supply walks-in at Apple and other retailers, as well as online orders.