some considerations
mebeuro, both pcs and macs can be fine machines for architecture-related software. i used to be an architect, so might be able to give a little insight on what to get. i have used both for architecture and what i do now - web design/development.
cad and 3d software are available for both. i would recommend looking into what software your school uses. that way you will be able to work on your laptop with the same software, and not have to worry about importing/exporting different formats or learning a different interface. this should not be your only consideration, but should be important.
i would say that with the current iteration of mac/osx, you will be able to run both mac and pc software so it will give you flexibility - but if you do use pc sofware on mac, before you buy mac, would do some research to make sure that doing windows virtualization via something like Parallels or using Boot Camp will be acceptable performance-wise with whatever pc software you use. i would think that specifically with the 3d modeling software, Boot Camp would be more viable since i think graphics drivers with virtualization are limited. but do the research and make sure. and you will be able to use mac software as well - so you can check out the various packages on both platforms if you want.
i would say, like in most industries, that most architects do use pc's, but there are still a good number of architects that rely on mac's. and there is some really good software on the mac too.
for me, i was mac in college, pc out of college, and went back to the mac recently specifically because mac can do both mac and pc - so have the means to work with both.
good luck in finding the right laptop - and your future as an architect.