So many creative librarians, but most library cell phone signs I've seen (and there are many on the Web) don't have much creativity, flair or humor. Most appear to be homemade: a cut and pasted unversal 'no' circle over a cartoon phone, printed, placed in a plastic holder and taped to the wall.
If a No Cell Phone sign is needed in the library (checkout desk, information desk, other?) why not a serious looking commercial sign like this one from www.victorystore.com or something homemade that engages your patrons?
I'm not working in a library now. The best I can come up with is the lame, "Don't make me come over there and shush you. Please turn off your cell phone while in the library." Surely the collective wit of librarians can come up with something effective and humorous. (I noticed that some libraries have gone the extra step and included exit signs reminding patrons to turn their phones on - a nice, user-friendly touch.)