I guess there is no fix, per AT&T Tech Support
They told me they've had lots of complaints but HTC has not given them an ETA when a fix will be available, and to go get a 3rd party messaging app until then.
Yeah, HTC really screwed us on this one. I've contacted them twice. The first time (early September) they said they were aware of it and a fix was being worked on. The second time (early October) they said they knew of no such fix.
Anyway, I've been using Handcent. I actually like it much better than the stock app. Even if they fixed the problem I would still use it.
There is a difference between a fix and a work around. Not everyone wants to have to install a third party app as a fix. Personally, I don't care for those third party apps myself. It is not asking to much to expect HTC to provide a fix, considering it is their software flaw. Unfortunately, it seems that HTC doesn't care, since it has been over two months since release.
I ended up downloading Go SMS Pro,as a workaround, so far so good although as already mentioned I shouldn't have to do this.
My guess is that HTC nor AT&T care as the Inspire is "old news" with all of the new phones coming to market.
I had that problem when updated OTA gingerbread, Got handcent no problems after that, it works great. Also got quikpic for disappearing pics. Or root and a flash custom rom. I now have 2.3.5 running 3.0 sense with no problems of anything missing or disappearing If you don't want to root get handcent.