about to start internship at a boutique firm, (think the likes of Monitor, LEK and OC&C), how easy is it to move on to a MBB when I graduate in a year?

An internship at one of them would stand you in great stead for MBB...

that's good to hear. just wondering if anyone's been through a similar situation. what happens if you get offered a job after the internship? how much time do you have before a decision needs to be taken usually?

You should expect to get a job offer and for them to give you a few weeks to decide. It has definitely been done in the past - I've seen loads of boutique interns end up at MBB and I've even seen some BCG interns end up at McKinseyMckinsey and BCG (and I think Bain) will fast-track people with "exploding offers" from internships. Just call up the recruiter and say I did an internship at place x and have an exploding offer, if I send you my CV can you get me through the process by such-and-such a date. I've seen people go from CV application to offer in 1-2 weeks when timings are really tight and the candidate is good enough.