I searched all over for a solution for a Citrix user with Office 2003, IE 6 and SharePoint 2007 who couldn't use datasheet view when trying to access a site with a customized master page. The browser would just hang without any error messages. To get out of it, IE had to be stopped from the task manager. I blamed Citrix for awhile - (which does bring its own set of issues for SharePoint users), but that wasn't it. I also found some great posts about adding code to the custom master page or changing the core.js:
But in the end, all I had to do was install IE 7 and the user was able to access the datasheet view on any site - custom master page or not, Citrix or not.