Google is bringing printing to mobile devices by extending Google Cloud Print to Gmail and Google Docs. It already exists for Chrome notebooks and will be rolling out over the next week or so in the US to devices running Android 2.1+ and iOS 3:
Imagine printing an important document from your smartphone on the way to work and finding the printout waiting for you when you walk in the door. Just open a document in Google Docs or an email in Gmail in your mobile browser and choose “Print” from the dropdown menu in the top right corner. You can also print certain kinds of email attachments (such as .pdf or .doc) by clicking the “Print” link that appears next to them.
The still relatively complicated, the setup requires a Windows PC -- with Mac support "coming soon."
Apple's iPad introduced printing with the most recent iOS update but it works with very few printers. Third party printing apps for the iPad are generally complicated (and costly). Enabling printing from smartphones and tablets in an easy and intuitive way will be a significant development. And if Google's the one that does it, that will probably boost usage of Google Docs in particular on mobile devices.