Database Attach breaks Alerts and the fix!
I got a query from a colleague about Alerts and went to test it. To my surprise, my test Alerts weren't working and after some troubleshooting I discovered it was due to my upgrade.
The fix is pretty quick - just download the txt file, rename to PS1 and call it with the WebApp url. It does little move than change the title of all alerts and changes it back. This triggers the alert to be rewritten and handled correctly in 2010.
For my example, my 2007 DB was attached and upgraded to https://VM-SP2010A, so my call was:
.\AlertFix.ps1 https://VM-SP2010A
HTH