ISBLANK(String) Returning false When String.length() Returns 0

I discovered this with version 21.0 of the Salesforce API: When a String's length is 0, the ISBLANK(String) Visualforce function will actually return false instead of true.

Visualforce Developer's Guide, Version 21.0 describes the ISBLANK function as follows:

Determines if an expression has a value and returns TRUE if it does not. If it contains a value, this function returns FALSE.

A search for ISNULL in the documentation returned no real results, which makes me wonder if ISNULL has been unofficially deprecated in favor of ISBLANK.

So, it appears that the only way to know for sure whether a String input has been blanked out is to use the length method and compare it to 0.