I don't see why it's unfair. If people with iPads can borrow all of these formats (they can, right?), why is it that much different? People who choose to read on computers can also borrow all of these things. Does that mean it was already unfair that people already could do this, depending on the device they had? However, we don't even know if this will be the case. I can understand the complaint that now epub users might face longer waiting times, but I don't really understand how other people being able to use something they can't use inconveniences them.
I was actually unaware that people with Sony/Nook/Kobo readers couldn't borrow PDFs from Overdrive. Why is this so?
I find it unlikely that Amazon will convert PDF files, because book PDF files often convert poorly, in my experience (page numbers, footnotes, title pages muck everything up). I think what will happen is that the borrower can get borrow a PDF, which the Kindle can read natively.