Can I use informal sources (particularly online sources)? I know that typically, unpublished references (i.e. references not published in some journal, especially online resources) are highly discouraged. I'm working on a paper regarding Bluetooth LE implementations, and most of the materials relevant to my paper are either online (i.e. on mailing lists discussing implementation of Bluetooth LE on Linux, or manufacturers documenting limitations of their Bluetooth LE chips) or in the Bluetooth 4.0 standard itself.

Is it appropriate to use these online references? They're pretty much my only source of information, as my paper is mostly on real-world implementations of Bluetooth LE.
If your paper is on "real-world implementations of Bluetooth LE" then mailing lists on implementations of Bluetooth LE and manufacturers' documentation of Bluetooth LE products are your primary sources.  This is akin to e.g., a historian using Napoleon's informal personal correspondence as a source for information on his world.

Of course, you should consider where these sources come from in how you use them. If you read on a mailing list or forum, 


  Bluetooth LE is the worst thing ever. Mine never works!


you would not use it as a factual source to conclude that Bluetooth LE is terrible. You certainly could use it as a source to indicate that some users have experienced frustration with early implementations of Bluetooth LE. (I cite bug tickets often in my own papers.)

Bad use of informal source:


  Bluetooth LE is not a good technology [1].


[1] "Let's all complain about Bluetooth LE here," Bluetooth User Forum, posted April 1 2104, http:/bluetoothforum/lets-complain, retrieved April 25 2014.

Good use of informal source as a primary source:


  Bluetooth LE users have expressed frustration with the technology [1].


[1] "Let's all complain about Bluetooth LE here," Bluetooth User Forum, posted April 1 2104, http:/bluetoothforum/lets-complain, retrieved April 25 2014.