Here are some stats/records/trivia that I've noticed from day one of the test between Australia and South Africa. In no way is this exhaustive, as there has been a lot of things happen on this day.
It is the second highest score by a team on the first day of a test, narrowly behind Australia's 494 against South Africa in Sydney in 1910/11
Clarke became the first person to score 4 double centuries in a calendar year.
Three players hit centuries on the first day of a test. I know that there has never been 4 players hit centuries, and haven't been able to find any other incident of a third century being completed on the first day.
Other interesting trivia:
It was Clarke and Hussey's fourth double century partnership, and second in succession.
Two South African bowlers (Morkel and Tahir) also got (less desired) centuries.
Two South African bowlers (Kallis and Steyn) had to leave the field with an injury mid-over. This is the first incident I could find of this happening twice in a day.