Here is the behavior...
1) These are mu-plugins.
2) If I take the sample code from the codex, and drop it into mu-plugins, I see this behavior. If I take other plugins listed as being compatible with WP2.5 and drop them into mu-plugins I see this behavior.
Behavior:
1) Menu items show up properly. Top level menu items, or sub menus.
2) When you select the menu item you get different behavior. In some plugin cases it works (not very often). But for most plugins it does not work. The sample code from the codec does not work.
3) Behavior Profile 1: The page header and footer areas of the admin page renders but the main content area is blank. This happens in the case of the sample code from the codex.
4) Behavior Profile 2: Some plugins will report an error about cannot load plugin and it displays the plugin file name. This happens in the case of the sample code in the codex for the "top level menu" example. But I also see this happen to other plugins on their sub menu pages.
Some debugging so far...
It looks to me like the pagehookname is getting formed differently between when the menu is rendered and when the page is "selected" and these pagehook names don't match, which results in the blank page.