Available now (/rolling out) for all Belle Refresh smartphones is a 'Fix for Messaging', offering 'critical fixes to known Messaging issues'. Quite what these are isn't known, but if you've been having SMS issues then give this a try and let us know if your problem has been solved. The update is small, at only 312k, installs in seconds, but does need to restart the phone in order to pick up the fix.
To check your own device, e.g. an N8, C7, E7, E6 or X6, find and tap on 'SW Update' on the main app menu, and then follow the prompts. The enforced restart is a slight pain, but then SMS is integrated into the OS at a fairly low level, so this isn't altogether surprising.
Comments welcome if you can shed light on exactly what this fixes!