Have the guts to hand out SHHH cards?
Coudal Partners, a design firm in Chicago, has a PDF that contains a set of SHHH (Society for HandHeld Hushing) cards that you can cut out and give to obnoxious cellphone users.

One says:
"DEAR CELL PHONE USER
We are aware that your ongoing conversation with _______________
is very important to you, but we thought you’d like to know
that it doesn’t interest us in the least. In fact, your babbling
disregard for others is more than a little annoying."

I would love to have the nerve to hand these out, especially when somebody is being waited on (at the dry cleaner or Starbucks or wherever) while still talking on the phone.

Get SHHH cards here.