I would propose that we put a blank icon file at /favicon.ico to avoid 404 errors from broken browsers not requesting the icon from the appropriate place. Blank because anything else would be "hard" for themes and users to replace, and not replacing a non-blank icon would hurt branding on some sites. There is a rather bloated blank icon file at http://www.sourceopen.com/favicon.ico but it can be made as small as 200 bytes with a little tweaking. I would be attaching my sample ico file here, hence the 'patch' status, but apparently we cannot attach ico files any more. Consider the linked file to be the submitted patch, although a shrunken version would be far more appropriate.
FAILED: [[SimpleTest]]: [PHP 5.4 MySQL] Unable to apply patch favicon404-174940-92.patch. Unable to apply patch. See the log in the details link for more information. View
FAILED: [[SimpleTest]]: [MySQL] Drupal installation failed. View
FAILED: [[SimpleTest]]: [MySQL] Unable to apply patch htaccess-favicon-404-fix-174940-drupal5.patch. This may be a -p0 (old style) patch, which is no longer supported by the testbots. View
FAILED: [[SimpleTest]]: [MySQL] Invalid patch format in htaccess-favicon-404-fix-174940_1.patch. View
FAILED: [[SimpleTest]]: [MySQL] Invalid patch format in htaccess-favicon-404-fix-174940_0_followup.patch. View