If you think you might be having issues with large lists in SharePoint 2007, there’s a good way to see if your WFEs are executing long running queries. Fire open ULS viewer, and filter on EventID 8sli. It will show you all list related queries that are taking more than 2 seconds. In each entry, you’ll see the duration, the list being called and the query that’s being called. While 2 seconds isn’t terrible it may indicate an issue. You should really pay attention to anything more than 5 I’d say. If you see a lot of these, think about working with list owners to correct their list views.
Large Lists Whitepaper – http://technet.microsoft.com/en-us/library/cc262813(v=office.12).aspx
ULS Viewer - http://archive.msdn.microsoft.com/ULSViewer