Like most things at Google, cloud printing started off as a beta back in April 2010 but Google is now launching it across mobile devices. Google cloud printing for mobile documents and GMail mobile apps is now a go. One way to check for it as Google noted is:
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.
Starting today and tomorrow English speaking users in the U.S. should start to see the options available to them. You'll have to set up your printer in order for it to work, but Google provides the documentation needed for that. Sadly, only Windows is supported forr the connection portion at the moment but Mac OS X and Linux support is coming soon. [Google Mobile Blog]