I haven't been paying much attention to the news lately, but my understanding is that the American President is on some sort of official visit here. While he was here he announced some sort of military alliance that would include U.S. troops training in Australia.
The main impact is that obviously this gives the U.S. a military presence within the region. It's also significant in that Australia is regionally closer to Asia but has been historically and diplomatically closer to America and the west. This is a bit of a slap in the face to China.
It also involves the fact that our current Prime Minister Julia Gillard, recently took over from fellow party member Kevin Rudd in some fairly dirty politics. K. Rudd has always taken a fairly warm stance towards Asia and been a little confrontational towards the U.S.
With elections looming in the near future and the possibility of a leadership challenge in her party I think that Gillard is trying to set herself apart from Rudd and show a difference in political stance that would justify her leadership coup. Plus being seen with as significant a foreign dignitary as Obama will increase her credibility as a leader.
Finally people have accused her of not achieving anything since she got into power and I think this is a way of showing her getting something done, particularly after her recent attempts to solve our immigration issues failed abysmally and made a bit of a minor diplomatic incident with Malaysia.