This is a great (and virtually free) way to pick up business. I don't see a downside.
Other possible downsides include, people coming up to you and bothering you with non-relevant "work" or "free work for equity" which you're not interested in.
There is also the exclusivity thing, where a potential client might perceive you as "too eager" for work, and you end up loosing a gig you might have gotten otherwise. (Granted, I'm reaching on this one).
I think it's a neat idea, but you have to be judicious when putting on any overt "label" on yourself or on the tools of your trade. I only bring this up due to my own experiences with stickers on my laptop. Getting them off is no easy task either (glue residue and such).
Maybe a scrolling marquee app. Heck, you could advertise whatever you want on your display at that point.
If it then becomes a trend, one could pay people small amounts of money for becoming mobile billboards on location enabled devices.
Next time I can't find a table to eat lunch at Panera the guy entrenched at a 4-person table for hours with a laptop and single coffee cup is going to try and make me read ADS? This... would be unwise.