Automatic message:
new field in DB uses to mean (99% chance of) it needs to be also handled by backup & restore. 
Manual message:
I really think we need to have one proper create_user() function ASAP in HEAD. Right now records are being inserted in the "user" table from a lot of places, and I'm not sure if are covering all them properly. Do externally authenticated users have this new field set? If so, how? With data from original auth store (ldap, db..) or following the rules above.
I know it's not critical for this field, but we need one function doing those bloody inserts and able to detect wrong situations and so on. Here it's the list of places "creating" users (huge one!):
admin/generator.php: $user->id = $DB->insert_record("user", $user);
backup/restorelib.php: $newid = $DB->insert_record("user", $user);
admin/uploaduser.php: if ($user->id = $DB->insert_record('user', $user)) {
auth/cas/auth.php: if ($id = $DB->insert_record('user', $user)) {
auth/db/auth.php: } elseif ($id = $DB->insert_record ('user',$user)) { // it is truly a new user
auth/email/auth.php: if (! ($user->id = $DB->insert_record('user', $user)) ) {
auth/ldap/auth.php: if (! ($user->id = $DB->insert_record('user', $user)) ) {
auth/ldap/auth.php: if ($id = $DB->insert_record('user', $user)) {
auth/mnet/auth.php: $remoteuser->id = $DB->insert_record('user', $remoteuser);
enrol/imsenterprise/enrol.php: if($id = $DB->insert_record('user', $person)){
enrol/mnet/enrol.php: if ($userrecord->id = $DB->insert_record('user', $userrecord)) {
lib/db/install.php: $guest->id = $DB->insert_record('user', $guest);
lib/db/install.php: $admin->id = $DB->insert_record('user', $admin);
lib/moodlelib.php: $DB->insert_record('user', $newuser);
user/editadvanced.php: $usernew->id = $DB->insert_record('user', $usernew);
Ciao 
Sounds like a good idea.
It should be called "timecreated" (inserted just before timemodified) and during the upgrade the field should probably be set to the value of the "firstaccess" field for all existing user accounts. Newly created accounts would of course have the value set to time()