Ilia Alshanetsky has officially submitted his logging patch for the mail function in PHP:

I've finally got of my ass and committed my mail logging patch I've written almost 2 years ago. This functionality is predominantly aimed at shared hosters that often have a problem identifying people who abuse the mail() function to send an in-ordinate amount of spam or hacked scripts used for the some purpose. The logging functionality is disabled by default but can be enabled on a per-directory or globally via 2 INI settings.

A new directive in your php.ini file (mail.log) lets you specify where the mail log needs to go. You can also use the mail.add_x_header setting to add in a mail header with the name of the script that sent it (and the UID).

It will be included in PHP 5.3 but if you're running PHP 5.2 and want to get a jump on it, here's the patch.