I zoom myself when a designer decides to use some stupidly small font and then often don't even notice the zoom is on until some other website is using a larger than normal font.
So, yes, people do zoom. Does it matter that this breaks your CSS3? I don't know, I'd have to see the breakage to know if it is a showstopper or merely a visual annoyance.
Don't be lazy and just think people won't. I was on a website the other day where the shopping cart, category and price filters all disappeared completely if you zoomed in, which was a shame because they had size 6 text and great products; it was enough to put me off buying anything from that website.
Text is usually not the problem, but images and thumbnails are. I rather zoom in to see the thumbnail up close instead of browsing to every site that is linked via one.
-bowerbird