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Do pages still appear on keyword searches?
Do pages still appear if you search for "a section of unique text in quotes"?
Are you still getting visitors referred by Google?
Have you made any major changes recently to navigation, filenames or major content changes?
Have you done anything - anything - that Google may consider, er, naughty?
If none of that offers any problems, then it shows what you probably knew already - page rank, for most purposes, really doesn't matter. If any alarm bells are ringing ... give us more info :)