The application Text Messaging (process com.motorola.conversations) has stopped unexpectedly. Please try again Picked up the Photon 4G for my wife about a week ago, and she's been getting this notification periodically ever since. Sometimes, it won't show up for days... Other times, it'll show up whenever she opens up the messaging app, or sends a message, or receives one, or lets it idle for a bit, or looks at it funny...

So far as I can tell, messages are sent and received without any trouble... Apart from this annoying notification. I have no idea where to even begin troubleshooting this. Any ideas?
As @Mathew Read already said, since your starting again clear the old app data.

Settings > Applications > Manage Applications > Select the Messaging App

Choose 'Clear Data' and also 'Clear Cache'.

If not you could also factory reset the phone.