As discussed in our forums, Treo 700wx owners have found they cannot send text messages to other carriers. This is because of the specific SMS encoding Sprint uses. Palm omitted support for this in the implementation of Treo SMS, meaning that the SMS messages are malformed when Sprint attempts to forward them to T-Mobile and Verizon.
Sprint and Palm are aware of the issue, and will have a firmware update available shortly to correct it. Until then, Sprint is issuing SMS credits for any user with an SMS plan.