[stormygal_indy]

Here's one:
Check out the following key in Registry: Hkey_Current_User\System\State\Messages\sms\Unread . The count value should be showing 1. Change that to 0 and soft reset.

Here's another I turned up for you over on SprintUsers:

Originally Posted by drue
There are several threads with various fixes depending on the cause. It seems to happen when using threaded SMS (or Codys Threaded) and a new SMS somehow escapes the software and sneaks onto the old sms stack. In that case, you can edit the following registry, setting it to 0. Then, when you click on the SMS icon on HTC Home, it should show the lonely unread text message.

[HKEY_CURRENT_USER\Software\Microsoft\Inbox\Setting s]
"SMSAvailable"=dword:1

0=old style
1=threaded style

Be sure to set it back to 1. On my phone, when I had to do it, setting it to 0 took effect immediately and no soft reset was needed, but I had to soft-reset to get back threaded-style messaging.