If you really want people in coffeeshops to approach you try something more personal and direct. "I'm xxxx. I build websites/applications, feel free to say hi,let's chat!" or whatever.
It doesn't mean anything; I bought this laptop & went directly to a conference, and just wanted it to look slightly distinctive, and the duct tape was all I had. It does cover the Apple logo so it looks like I hate Apple or something (I do hate excessive and ostentatious branding).
I kept it because I kept having conversations about it. Even baristas remember me as the guy with the X.
It's odd but I don't think a logo has any meaning any more, because real life turned into Nascar.
If I saw that, I would probably have assumed you were just a huge X-Files fan or something.
Actually, thinking about it now, I think the next laptop I buy, I'm going to put an X like that on there, along with a small sticker that just says "I Want To Believe".
Well, that's one problem I won't have to worry about. :-)
I would question, however, whether putting a sticker of any kind is gonna help you score gigs. I mean, if you were looking for a coder, would you really approach some random guy/gal in a coffee shop based solely on a sticker on their laptop and knowing nothing else about them?
Maybe having a sticker with a link to your github, as suggested by others, would be more useful (if your user id is memorable).
I full sentence briefly explaining what you do, and more importantly giving people permission to interrupt you would stand out more than just some random sticker.