It really depends on who you work for. When I was working at a startup, most of my side projects weren't related, but when I was working at Yahoo and Facebook, well it's hard to do something interesting on the Internet that's not related to Yahoo, and maybe Facebook (they're not as broad as Yahoo was, but they could potentially claim a lot). It doesn't matter if it's something way outside of your job area, or even if it's something they would never let you do at work, if it's in their sphere.