This is happening a few times in the past 2 weeks. Someone will send me a text message, I get a notification and see who it is and the beginning of their message, so I click the notification to check the message itself - and it's gone.
It's not like my inbox is full or anything - I still do receive some messages correctly. But for example just now, a friend text me to ask a question, and when I went click the notification to reply, the text just wasn't there.
I'm using Handcent SMS, but I also checked the regular Messaging app and these particular texts are not available there either (which I guess makes sense, as don't they both pull from the same internal text msg database?).