Does your list contain folders? Do the folders have different permissions than the list itself? I will explain my madness. I have a list with folders. All users had read permission at the top level of the list but I didn't want them creating/editing items at the top level. I wanted them to create/edit within the folders only. So, at each folder level, I assigned contribute permission for the appropriate groups of users. All worked well through the SP user interface (users could only work on content within their assigned folders). Now, SPUpdateMultipleListItems operates at the list level. So, when my users were running an event that required the SPUpdateMultipleListItems method, all indications were fine from a code trace process but the reality was, the update wasn't occurring because the users didn't have contribute permission at the top level. As soon as I changed that permission, everything worked fine. Also, the operation still respects the permissions set within the individual folders. For instance, if a user has permission on folder "A" but not folder "B" and each folder contains an item with a Title of "Analytics", that user could run the SPUpdateMultipleListItems per your code/query above and it would not touch the item in folder "B." However, if the user doesn't have edit permission at the list level, they update nothing, regardless of the folder permissions.
Geoff
Geoff