tmux conf stuck

[debian@waysoft.com]

I *know* I'm missing something obvious here...

Debian squeeze, XFCE, up-to-date

Trying out tmux (stock version installed via aptitude)... Didn't like
C-b as the the escape key, so tried backtick as I've used with GNU
screen...

~/.tmux.conf
  unbind C-b
  set -g prefix `

Backtick worked fine in tmux, except that I could not figure out how to
send a backtick to an application.  Double backtick, as works in screen,
didn't work, nor did any other escape strategy I could think of.

So... on to other tmux options.  I changed ~/.tmux.conf to use C-a.

~/.tmux.conf
  unbind C-b
  set -g prefix C-a

Ctrl-A didn't work, but backtick still did!

-I tried to put an obvious error in ~/.tmux.conf... undetected by tmux.
-I tried inserting an "unbind `" into ~/.tmux.conf... backtick tick
works.
-I tried deleting ~/.tmux.conf... backtick still works!
-I tried tmux -f tmuxconf, where tmuxconf enables C-a... backtick still
works!!
-I tried removing .screenrc, in case tmux looked at it... backtick still
works!!!
-I tried turning swap off... no change.
-Nothing obvious in the environment, or /tmp.

So... why can I not get rid of the ghost of the old ~/.tmux.conf???

Thanks for any help,
Jeff