I was working with a customer who ran across an issue where when they'd go to their groups administration page, the list wasn't paging. There were more than 100 groups on the site(which will cause poor performace), so the list pager control should have shown up, but it wasn't there. It turns out the customer had less than 100 SharePoint groups on the site, but they had added domain groups to the site as well. (Add domain groups to SharePoint groups PLEASE) For some reason, SharePoint didn't count the domain gorups towards the 100 it needed to page the list. It was only counting the SharePoint groups. This caused the pager not to show up. When we removed the domain groups, and added a few more SharePoint groups, the paging control was visible.
(To get to the page I'm referring to, groups.aspx, from the site actions menu, select site settings --> People and groups. Then click either the groups heading, or the more option in the list of groups on the Quick Launch)