Yes, I know the title is terrible, and you need to be of a certain age to remember those fine TV adverts for the Citylink coach group.
As of last December, Citylink have introduced wifi on their buses between Glasgow and Edinburgh. It gets better: it’s free! I’ve not tried it yet, but as soon as I’m heading that direction by public transport I will. Here’s some more about the technology being used.
Update: National Express also offer free on-coach and in-lounge wifi.