This one threw me for a day or so. When using a GridView in a web part or page, the Update command of the GridView was occasionally being raised, but mostly not. Everything else worked as expected.
After a lot of hair tearing, I decided to install the March Public Update. Lo and behold, it suddenly started working.
So, just so as I remember this painful experience, that if I am having strange behaviour with ASP.NET controls in SharePoint, to make sure that I have the latest patches on the Server and SP installed.