A Steam Community Announcement notes that a Linux beta of Portal 2 is now available without much fanfare (thanks Ant via Slashdot). Word simply is:

We have just made Portal 2 available in Beta form on Linux; to access it, you need to opt into the Beta from the Portal 2 properties in your Steam Library.

We look forward to your feedback; head to our dedicated tracker to report issues and look at know issues:

https://github.com/ValveSoftware/portal2/issues