It's always nice to know you're not alone. I always pat the outside of the aircraft as well, but was never brave enough to tell anyone that I did so.
Gibon, Smoketrails and Jackieofalltrades - I salute you and wish I was on the same flights as you.
The only additional piece of wierdness in which I indulge is that I sit quietly and pay attention to the safety briefing, rather than read/snore/talk loudly/look around like I'm too seasoned a flyer to pay attention.