Reading the lyrics (from mobile / ipad / book etc) is equally disrespectful - but that's another thread...

Really? I get ribbing from the Raise The Roof lot, but as YorkshireYankee pointed out, it's just a different type of paper. I'll always have my phone on me (on the off chance that some big company email me saying that they like my writing and want to give me lots of money. I'm a dreamer) and I've left paper copies at home before (*cough* and I'm too much of a cheap git to pay 5p at the IC to print them off *cough*)

I don't see how using your phone to sing from is any less disrespectful than singing from a piece of paper.