What does the blue I icon next to my custom display form mean?
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The page has been customized (un-ghosted) and has been disconnected from the master page. Any changes to the master page will not apply to that page.
This happens when you edit the page in advanced mode and change stuff other than web parts, for example create additional web part zones (or many other conceivable things).
See here for details. The article is about WSS 3.0 but applies to MOSS 2007 as well.
[Edit:] corrected ghosted to un-ghosted as Per suggested.