I had a scare last Friday when I thought that some pages in an application that I had already deployed to a production SharePoint site were failing due to a large number (3300+) of items in a list. Turns out that it wasn’t the problem, but it’s worth understanding that the party line says that there shouldn’t be more than 2000 items in a list container. (By container, we mean the root of the list or any folders contained within it.)
Once you’ve exceeded the 2000 item limit, you may see this message on the list settings page (preceeded by an ominous red exclamation point):
This list or library contains a large number of items. Learn about managing a large list or library and ensuring that items display quickly.
As is frequently the case in MOSS, the link only takes you to the home page of help, and I couldn’t find anything inside help that gives more detail (though it might be there somewhere).
The white paper titled "Working with Large Lists in Office SharePoint Server 2007" I found on the Microsoft SharePoint Products and Technologies Team Blog gives a lot of detail about how to think about all of this.