Status (by robm at Mon Mar 15 01:33 UTC)
There are currently problems delivering SMS messages to US phone numbers. This is due to problems at our SMS gateway provider.
http://support.clickatell.com/us_notifications.php
Many US carriers allow you to send an SMS to a user just by sending an email to their email-to-SMS gateway address, so you can try using that instead.
http://en.wikipedia.org/wiki/List_of_carriers_providing_SMS_transit
