I realized the problem when getting reports that phab didn't send emails. Looking in the logs, I was seeing:
[30-May-2014 10:33:48 UTC] [2014-05-30 10:33:48] EXCEPTION: (PhutilProxyException) Error while executing task ID 316199 from queue. {>} (phpmailerException) Invalid address: Manuel Klimek <klimek@google.com> at [/srv/http/phabricator/externals/phpmailer/class.phpmailer.php:462]
[30-May-2014 10:33:48 UTC] #0 PHPMailer::AddAnAddress(to, Manuel Klimek <klimek@google.com>, ) called at [/srv/http/phabricator/externals/phpmailer/class.phpmailer.php:408]
[30-May-2014 10:33:48 UTC] #1 PHPMailer::AddAddress(Manuel Klimek <klimek@google.com>) called at [/srv/http/phabricator/src/applications/metamta/adapter/PhabricatorMailImplementationPHPMailerAdapter.php:64]
[30-May-2014 10:33:48 UTC] #2 PhabricatorMailImplementationPHPMailerAdapter::addTos(Array { 0 => Manuel Klimek <klimek@google.com> }) called at [/srv/http/phabricator/src/applications/metamta/storage/PhabricatorMetaMTAMail.php:611]
[30-May-2014 10:33:48 UTC] #3 PhabricatorMetaMTAMail::sendNow() called at [/srv/http/phabricator/src/applications/metamta/PhabricatorMetaMTAWorker.php:26]
[30-May-2014 10:33:48 UTC] #4 PhabricatorMetaMTAWorker::doWork() called at [/srv/http/phabricator/src/infrastructure/daemon/workers/PhabricatorWorker.php:85]
[30-May-2014 10:33:48 UTC] #5 PhabricatorWorker::executeTask() called at [/srv/http/phabricator/src/infrastructure/daemon/workers/storage/PhabricatorWorkerActiveTask.php:123]
[30-May-2014 10:33:48 UTC] #6 PhabricatorWorkerActiveTask::executeTask() called at [/srv/http/phabricator/src/infrastructure/daemon/workers/PhabricatorTaskmasterDaemon.php:19]
[30-May-2014 10:33:48 UTC] #7 PhabricatorTaskmasterDaemon::run() called at [/srv/http/libphutil/src/daemon/PhutilDaemon.php:85]
[30-May-2014 10:33:48 UTC] #8 PhutilDaemon::execute() called at [/srv/http/libphutil/scripts/daemon/exec/exec_daemon.php:112]
I haven't had time to fully look into the problem yet, but it looks like ValidateAddress started to go into the branch where it uses php's filter_var function, and that rejects the normal looking email addresses.
I quick-fixed it by making all exits from that function return true, and things seem back to normal.