A kind soul on AO3 brought this issue to my attention yesterday and, since I haven't seen it appear on my LJ flist yet, I thought I would signal boost the information.
There is a website called Ebooks Tree that has been taking ebook files from various places - including AO3 - and putting them up for 'free' download for their paying customers. This is happening to published authors as well as fanfic writers, so it's kind of a big deal, but AO3 is advising any writers whose work appears on this website to submit a DMCA complaint in the hopes that it might galvanize the website to do something.
There is more information about how to go about doing this here and here.
*heavy sigh* The Internet is such a bullshit place, sometimes.