According to 9to5mac, the iPad HD (or 3) will be available on March 16th. Last year's iPad 2 release date was March 11th, which was the Friday following the announcement event. The 16th this year would be the same.
Sean Gunn on Working With His Brother
In addition to the following-Friday bit, two new Apple stores in London and Houston, Texas will be opening on the 16th. 9to5mac also says that Apple won't have a pop-up store at SXSW this year, like it had last year. It also makes reference to some kind of possible Apple event next week, but there are basically no details about that right now.
To find out all things iPad HD as they're unveiled in real time, ride along with us on our iPad HD Liveblog this Wednesday, starting at around noon EST. [9to5mac]