I won't say it's common, but it isn't all that unusual in the US either, at least among my group of friends. I came in with only 20-something hours and was on the low end (really focued on my math, finished all the necessary credits for my Comp Sci. degree before leaving high school). A friend of mine, on the other hand, came in with just under 70 credits, but they were all the gen-ed with some upper level math thrown in. While that'd normally be a little over half a degree, she added a second major that balanced it out.

A number of my friends did this though, so I don't think it's quite as big of an accomplishment in the US than elsewhere. Though I'll be the first to admit that my group of friends tended to be the high achievers in high school/university.

-- A undergraduate in CS/Mathematics