She's saying she might run for two reasons, assuming she actually means she might run.
1) Speculation is constant news, if she says "I will run" right now it becomes news for a few days and then people forget about it. It losses all the excited which is more useful later during election time than it is now. So as long as its a "maybe" every time she does an interview and the interviewer asks her about her presidential run, she can drop hints or more maybes which will get onto the news. Or better yet maybe stronger hints as election year gets closer.
2) In case something happens between now and 2012 that ruins her reputation, whether among her supporters or other Americans in general, she can back out with less embarrassment and salvage her current role as a speaker/book seller/celebrity, etc than if she came outright and said she'd run.
Of course if she's not really planning to run at all, than its just for media attention.