Do any Mexican bus lines provide Internet access?

According to Mexperience.com, some buses do indeed have wifi onboard, but generally only in Executive Class buses:

ETN's buses now offer WiFi on some routes. The service is free and enables you to send/receive email, surf the net and make internet-based phone calls from the bus using a service like Skype. Service quality varies and, depending on where you are geographically, internet signals may not be available at certain points en-route as many of Mexico's highways traverse mountains and other wild terrains.

So what this likely means is the bus has a 3g signal that is hooked up to the wifi, meaning you'll be sharing all the internet with anyone else onboard. If you're the only one - lucky you!

Another blog post on the same site describes the experience and would seem to indicate that ETN has had onboard internet since 2008.

ETN stands for Enlaces Terrestres Nacionales, and they have a website. To see which routes currently offer Internet, on their opening page, hover over the 'Servicios' tab, then click 'Internet a Bordo' (or switch to English, then 'ETN Services' followed by 'On Board Internet').