This is just a ploy by the Kerry camp to make some people think they are seriously considering Nunn, in an attempt to bring in some southern/moderate voters who might remember this consideration come election day. A few more such names will pop up in the coming weeks of a similar nature, but none (as with Nunn) are really in the running for VP.
No one will remember who was rumored one week after the real guy is chosen except fanatics like us.