Hey there, I'm applying to American universities for 2009, and I'm at the end of the enormous process which has taken half my holidays.
Just wanted to butt in to clarify some stuff; thought it might be helpful since I've spent months reading up on this stuff and when I started trying I had absolutely no clue what to do.
First you ought to know that at the biggest colleges - and the more generous smaller ones - admission is needs-blind, and all demonstrated need is provided. That is, they'll let you in regardless of if you can pay, and then they'll give you all the money you need to make it possible for you to study there. For these nice ones, it's INTERNATIONAL needs-blind too. Princeton, for example, pay on average 70% of each student's tuition and board. Princeton Harvard Yale Georgetown are among the best; Berkeley I've heard is good too.
You don't have to do the IB at all. Get good marks at school and submit your HSC/VCE/whatever you're doing.
On top of that you need to sit two exams - in Sydney they're run at Macquarie University on Saturday mornings monthly throughout each year.
The SAT I is a reasoning test, which goes on for quite a while; theoretically you don't need to study for it - it's said to test natural ability through easy maths/reading/writing - but it's good to do a little preparation.
The SAT II is comprised of subject tests, not unlike your school exams, and most top colleges demand you do three of them. The syllabi are often a little different to what you get here (I had to go learn pretty much a whole new Phys course the other week), but it's not too hard.
Other than that, you need teacher references and a rocking CV.
Extracurriculars are very impressive, but they don't want some cynical resume-obsessed dilettante - show you're passionate about a couple things and be deeply involved in them. Being a part-time member of eight clubs won't make you look appealing.
The colleges then often organise interview with alumni - who are lovely and not at all scary - and then you fill in your applications for financial aid.
Come late March, offers are sent to you along with how much money they'll offer, and you make your decision by May.
Then term starts in September/October.
Don't let yourself get talked out of it if it's what you want to do: it's potentially a life-changing opportunity, and the education there is world class.
If ya wanna talk to someone who actually knows their isht (ie not me), get in touch with education USA via the US embassy. They have tons of advisors and resources and things.
Good luck with it!