There seems to be a lot of issues where people all of a sudden cannot send text to a particular contact (or a couple of contacts). I've found couple of these cases are related to Facebook contacts. With the option to sync contacts with your facebook account, many times the information in the mobile phone field in a person's FB profile is incorrect or not in proper format. This causes the message origination to fail on the device never making it to the network. As a result when you call your provider they will have no record of any such msg origination attempt.
One oddity I found is when the mobile phone field in the FB profile has a 1 in front of the area code and number the device is reading this as an international number and prepends the terminating number with 011 - also causing the message origination to fail.
In my friend's case... she could not send to one person soon after getting the Droid X. "Message not sent" was the error message in the notification bar on the top of the screen. She would delete the msg thread and the contact. Create a new contact using only the 10 digit phone number and send a new message. That msg would be received by the other party, but as soon as that party replied, her contact would change for that person to 1xxx-xxx-xxxx. This person was in her FB contacts and it was set to sync. Upon looking at the other party's FB profile, their mobile phone info was set to 1xxx-xxx-xxxx. We had the other party edit their FB profile to only have the 10 digit mobile phone # and the problem went away. (Note they had to do this from their PC as they could not change it from the mobile FB app)
Seems to me like Google and Facebook will but heads over who's fault this is before there is a fix for it. This worked as a workaround for us.
Just something to be on the lookout for if your problem sounds similar.