I'm not condoning this, but Twitter being replicated in real world graffiti is perhaps the ultimate sign of 'mainstream-ness'?
Especially when you see the follow up:
Questionmarc's Nottingham Twitter Graffiti (with irate follow up)
From Questionmarc via Urban Prankster
Follow me on Twitter here - @nickburcher
Twitter demographics and user numbers
Twitter in UK newspapers - micro-blogging hits the mainstream
Banksy's virtual ghetto in Second Life
Banksy's Katrina tributes go viral.......
....and get stolen