A-Levels to MIT / Berkeley

Well, regardless of anything, getting into MIT or Berkely will be quite difficult, especially an international Student.
You will need to take SAT I, and three SAT IIs, one of which must be Maths Level IIC. Taking an interview with your local MIT alumini will help. Pay particular attention to your essays and get excellent recommendations from teachers and your councellor/principal. You should also have many extracirricular activities.

For MIT, showing examples of your interest in your field is a good idea - a published research paper, an extensive project, etc.

Really? I know quite a few people who applied/ are applying to Ivy League schools and such which have done things like that, and it's definitely helped their application (in terms of those successful, and those not). For MIT, I would say science projects that went far with well developed papers will be a definite plus, especially if they're sponsored by a college [mine last year was, for example]. I know people who've done huge research projects at colleges during the summer and presented them, which is another good example. My friend did one at a Agricultural Governor's School during the summer - she's going into veterinary studies. Of course not everyone has something like that, but I'd saying having unique accomplishments is a huge part of the process, in terms of being successful, and just showing them what you're passionate about - whatever that may be! Also, being British I know that the hype over ECs is a very american thing, and wanted to let the poster know that standing out beyond grades will be very important. This is just my experience, anyway.

You can often join in summer programs at universities/research institutes and work alongside professionals, (PhDs usually) and conduct research. The research doesn't have to be somthing big like the theory of relativity, and you can simply submit is as a co-researcher.

We live in the US. You're right getting a published paper is not likely. More often, you can attend a University summer program, conduct some research with one of the professors and make a report on the the research. This report is viewed quite highly by institutes such as MIT.

you need ECs (not 'president of ___' but something really impressive)

No offence taken.

Like Aiman pointed out, the US is quite different in terms of what's done especially during the summer by ambitious high school students. At the undergraduate level research work is encouraged. In fact, during my admitted students tour of a college in VA a chemistry professor actually encouraged freshman to join a research group during the summer and get paid to contribute to a post graduate's research work. He said that a few students had even branched out on their own and written papers on it.

I doubt many people have made discoveries quite as ground breaking as the Theory of Relativity, and I certainly doubt the papers are anything like as in depth as a phD dissertation, but some of the work that select students do is quite impressive and can help an applicant to places such as MIT in the US.

I doubt many people have made discoveries quite as ground breaking as the Theory of Relativity

but some of the work that select students do is quite impressive and can help an applicant to places such as MIT in the US.

So, take it with a pinch of salt, I suppose. In a way it's ridiculous that ECs can mean so much when you're applying to an academic institution, but with approximately 26,000 applicants they have to narrow the pool down somehow.

silkweed is quite right, I have to say. I applied for Electronics Engineering, had some wonderful recommendations from both my teacher and councellor. My essays were quite good as well, and my SAT scores weren't bad. I had won numerous awards and was a very active extracirricular participant. I founded my schools' Students Association, along with other things.
But i still didn't get in . I didn't call them up to ask why my application had been rejected, though. I can't really see any shortcomings, (other than perhaps not submitting a research paper), so I assume its to do with luck. As silkweed mentioned earlier, they do have a very large number of applicants, so I some-what doubt that they'll let you in if you don't show anything you excel. I hope I didn't discourage you or anything.

I'm going to go a little off-tangent here but I'd like to ask how students from th humanities stream can contribute to the publicshing of such "research papers" exactly? Where can we find such resources or opportunities? It does seem very elusive to me and will certainly impress me a whole load if an undergraduate can co-author or in any meaningful way contribute towards the publishing of a research journal!

These opportunities basically don't exist. I defy anyone to show me a paper to which any undergrad has contributed.

This sort of thing is purely jumping through hoops, and illustrates what's wrong with the US system; it's just about they length of the list of work placements and societies that your highly-placed mummy and daddy, and exclusive private school have encouraged you to pursue.