It's neat to see this one getting talked about. Check out Dean Esmay's original post on the subject, then look at Ace of Spades and associated comments. When I've posed this question to really smart associates, it's unbelievable what an argument it can turn into before you can get them to see the right answer. I still like to ask folks this one. Serendipitously, this is a topic right when I'm getting back into some heavy-duty self-study of calculus, transfinite numbers, and the whole question "what is a number". Why am I doing that? Because I originally learned a lot of it half a lifetime ago, it is beautiful, and I want to "get back into it".
Ace has another puzzle post here.