This is probably false. It certainly is a crazy theory. My evidence only consists of stuff that Mr. Google has brought me.
HOWEVER...
I believe there is a chance Zell Miller "took one for the team", and the man that convinced him to do so was Bill Clinton (with an assist by our good friends James Carville and Paul Begala-who had run campaigns in the past for both men).
Here's the theory:
Zell Miller was the keynote speaker at the Democratic convention in 1992.
The 1992 Democratic covnention was held in New York City.
When the Republicans decided to hold their 2004 convention in the very same building the 1992 Democratic one was held, Bill Clinton, who's office was not far from both the convention and ground zero, felt disgusted at the obvious political ploy of exploiting 9/11.
So, he thought back to his convention and Zell.
He realized the irony of having him be the keynote speaker again, in the same building, 12 years later, but this time for the other side, and knew that the GOP would love the irony of it, and wouldn't be able to resist it.
Zell Miller was a very conservative Democrat who could give a crazy southern-preacher style speech when needed, and was known to do wacky things (his nickname was already "Zig Zag Zell" at this time).
He was retiring in 2004.
Clinton convinced him to take one for the team and go DDEEEEEPPP undercover.
So, in planning for this, he became a pseudo-wingnut, and even wrote a book.
The wingnuts fell for it hook, line, and sinker.
Then, on convention night, he made sure to give the most over-the-top speech possible (with those in the audience eating it up and hooting and hollering and looking crazy too), and then afterwards went crazy batshit on both CNN and MSNBC. His speech helped drive away undecideds from George Bush in droves.
Guess who ran Zell Miller's campaign for governor in 1990?
James Carville and Paul Begala.
Oh, and he has been a liar before...looks like I'm not the only person whom this crazy thought has occured to (I found this link as I was preparing research for this diary entry):
http://greenehouse.net/archives/2004/08/great_moments_in_zellotry
If Carville, Begala, Clinton, and Zell himself did thier jobs right, we will never be able to prove this (except if one of them prints a HAHAHA YOU FELL FOR IT book thirty years from now).
However, we can see if the timeline works.
Ok, the theory is that Zell is a "9/11 changed everything" type, who has been wandering off the reservation ever since then.
The Republicans announced thier convention in early January, 2003:
http://www.2004nycgop.org/news/releases/010603.shtml
So, the goal here is to find a vote or speech that would be uncharactristic of a (slightly wacky) conservative Southern Democrat but characteristic of a Republican, or Zell insulting or attacking the Democratic party, or Kerry, or Dean, etc., before January 2003 but after 9/11.
He endorced Bush in October, 2003, and published his book at about the same time.
http://www.weeklystandard.com/Content/Public/Articles/000/000/003/309nqnas.asp
This is when it became obvious he was no longer a Democrat in anything but name, although it took a few months to ramp up.
Looking at his voting record here:
http://www.ontheissues.org/Senate/Zell_Miller.htm
...I see nothing that really disproves my theory.
He made several pro-Democratic votes between 9/11 and 1/03:
Voted YES on adding sexual orientation to definition of hate crimes. (Jun 2002)
Voted NO on require photo ID (not just signature) for voter registration. (Feb 2002)
Now, he also made anti-Democratic votes prior to 9/11, which is why his turncoat trick works:
Supports anti-flag desecration amendment. (Mar 2001)
Voted YES on repealing Clinton's ergonomic rules on repetitive stress. (Mar 2001)
Here is the entire section on "Zell Miller on Principals and Values". Notice the huge gap between Jan 2001 and Oct 2003.
For Bush because "my family is more important than my party". (Sep 2004)
We need bipartisanship in times of war. (Sep 2004)
Democratic party is value-neutral and misguided on issues. (Oct 2003)
The Democrats in power don't care about the South. (Oct 2003)
Democratic party leaders too liberal for the South. (Oct 2003)
Member of the Senate's "Centrist Coalition". (Jan 2001)
Member of Democratic Leadership Council. (Nov 2000)
New Democrat: "Third Way" instead of left-right debate. (Nov 2000)
Member of the Senate New Democrat Coalition. (Jan 2001)
It really looks like October, 2003 is when he made his big turncoat splash.
Here's some things he did in late 2002:
Here's a quote of him defending Max Cleland in an ad just prior to the November 2002 election(although Chambliss's chief strategist thinks it backfired).
http://atlanta.bizjournals.com/atlanta/stories/2002/11/18/editorial2.html
Here's a very wishy washy endorcement of the Iraq war in Sept. 2002. He states he's all for it, because as a former marine he was taught to say "Aye, aye, sir,"...but people in his home state have a few...um...questions...
http://www.washingtonpost.com/wp-dyn/articles/A48684-2002Sep6.html
Here's an interesting thing in the offical Kerry blog that has him supporting Cleland in 2002 for the exact same thing he was attacking Kerry about:
http://blog.johnkerry.com/blog/archives/001628.html
There is much evidence that Zell was always barely a Democrat (at least since 1994 when he almost lost a race for governor). But this was true both before and after 9/11. However, I will admit that this may just mean he was wavering for quite some time, even prior to 9/11. As I started the research into this diary entry (and posted some of it in an open thread), I became more and more convinced-then less and less. Zell has done some really wacky things over the years, the least of which has been voting against his party on many occasions.
Like I said, it's a crazy theory. It may very well be wrong. But remember-he's still a Democrat.