Starting off at 15 items per page, it shows:
Changing to 30 items per page, it still shows:
But changing to 50 items per page, it disappears:
closed as too localized by Emmett Jan 18 '13 at 23:14
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It looks to me like the problem is not the number of items per page, but the number of pages.
The critical amount seems to be 2. If there are 2 or less pages, the items-per-page links do not show up.
This also affects SuperUser and Meta.