Touching with the nose can also be an incredibly submissive gesture, and is seen commonly in pups. It's a soliciting gesture; they're asking for food.
Now, typically this kind of touching would happen around the face/lips (i.e., where the food comes from). Adult dogs will do it to another animal as a sign of submission.
Our adult GSD does this all. the. time. She's incredibly submissive, and is contantly poking at poor Gracie (the dominant dog), and us humans (never Heidi though).
The poking you're describing, on the hands/arms while you're sitting, is a different kind of solicitation...he wants something else, probably pats or just plain old attention. This kind of thing, in an otherwise calm, stable dog...wouldn't bother me at all. In a bossy, dominant dog, however, I wouldn't reward the behaviour, I'd ignore it. I would instead call the dog to me and initiate the positive interactions myself.
Solomon - black DSH - king of kitchen raids (11)
Gracie - Mutterooski X - scary smart (9)
Jaida - GSD - tripod trainwreck and gentle soul (4)
Heidi - mugsly Boston Terrier X - she is in BIG trouble!!! (3)
Audrey - torbie - sweet as pie (11 months)
Patrick - blue - a little turd (but we like him anyways) (6 months)
Boo, our Matriarch (August 1 1992 - March 29 2011)