Its simple, and a few people have mentioned it, but several other people come up with off the wall responses (to be funny, I hope).
My theory, is that many people at the coffee shops are students. They usually have the time to sit down and work on stuff, where as a lot of the time, business people don't have that. Some do, but those usually are from companies that have the 'whatever it takes to get your work done,' type of companies, which are flexible and allow their employees to go to a coffee shop to work.
I have been to a couple of coffee shop type places with my mac (and with friends just hanging out), and there are a lot of macs. But usually, there are college students hiding behind them. Those students are the same ones that could be sitting in my classes with me, or in the library. I see a lot of Windows computers when around campus... But I see just as many or more macs.
The other possibility, as one person mentioned, is you notice them more now that you have a mac. I remember when people were 'prejudice' against macs to an extreme extent. For me, I like to see someone else that understand where I am coming from, in terms of the advantages over windows, crappy hardware retailers, etc. Its the same way with my iPhone. Once I got my phone, I started noticing people with iPhones more often.