you're going to get several people recommending a basic steel frame. i will too, though i've never ridden aluminum. also, well-built wheels are going to be what you want in order to withstand impacts - IRO (www.irocycle.com
, search the forums for more info about them than you can handle) gets their wheels handbuilt by velocity. there are a lot of cheaper wheels out there with similar hubs and rims, but a lot of those don't come with desirable trueness or tensioning, and that's a trip to the bike shops and twenty bucks or so to straighten that out.
there are lots of affordable bikes that can take this curbhopping, stair-taking beating. my IRO Mark V does it quite well; search for the thread about budget bikes (i think it's called "a discussion of budget single speed and fixed gear bikes" or something like that) for lots of information about which ones and some of the pros and cons of each.
i find bunny hopping tough, but curb hopping no problem - lift the front end and then lift the rear end. not exactly one fluid motion, but smooth enough and it gets me off the street and on to the sidewalk when i'm dismounting.