Thanks everyone for your quick replies.
Bunce: No, I'm not opening in a new window. I guess this is a possible way to work around the problem, but it still seems like the browser back button should remain enabled.
Diep-Vriezer: I could create a back button, but again, it seems sort of unnecessary when the browser has a back button that should be working.
Anyone have any ideas why the back button becomes disabled? I noticed that the button does not become disabled if the list in the datagrid is short (under 200), so perhaps this is an issue of memory reserved for the browser's cache. Is this a possibility?