Retroactively change email signature?

["Stephen Sinclair" <radarsat1@gmail.com>]

Hi,

I imagine that changing old commits is considered one of the great
evils of source control, but I have a small problem:
I have a habit of testing my code on several different computers and
operating systems.  Sometimes when setting up a new computer for
testing (and thus, for development, since testing always results in a
few commits), I have forgotten to use git-config to configure my name
and email address.

Since git doesn't warn when I've forgotten to do this, my git-log is
now sprinkled with various email addresses, some which are just my
username@hostname.  This is completely useless to people who might
view my public repository.

Is it possible to retroactively change the author and email of several
commits?  Perhaps some sort of search-and-replace for the commit
metadata?  Even for older commits, I'd like to change the email
addresses to my current address.

If it's not possible it's not the end of the world, but I thought it
wouldn't hurt to ask.

thanks,
Steve