Let me add a few words to the discussion...
Mercy was indeed ripped from a CD that Bono gave someone while driving around in a car with them. I haven't heard that it was a full album, that in fact it only had a couple of songs on it. One of their "work in progress" CDs that seemed to be prevalent during this recording session.
The CD that was stolen from the photo shoot this summer was another one of these. Four album tracks and "Mercy" were on that release as well. It was actually stolen, and it was narrowed down to a handful of people who had access, so it's doubtful that anything will ever show from that test CD.
The album that got leaked in mp3 was a full blown version of the album, and was the finished album as we received it. There are no differences. Whoever this RNS group is who are ripping these things, seem to have access to Universal's early promo releases, or the production plant in which they were made.
Finally on Fast Cars - the only people who got this as a bonus track on the regular versions of the album were Japan and the UK. Far from just the USA got shafted on that one. The rest of the world got "Fast Cars" in the box set, but not on the single album, or the regular album / dvd version.
The reason the UK and Japan get special treatment? Import CDs are actually cheaper in both locations than buying the domestically made CDs. You can buy an import from Europe or the USA cheaper than buying the one made in the UK. Thus to help sell more domestic CDs, U2 has the habit of putting an extra track on the releases from these areas...it's been happening since the 90s.