j.l.String.format specification says about passing null as format argument: "The behaviour on a null argument depends on the conversion". Conversions described in j.u.Formatter specification. According to this specification only "general" conversions ("%s", "%b" and "%h") may be applied to null argument. If another conversion have been applied to null, method should throw NullPointerException according to j.l.String spec ("Unless otherwise noted, passing a null argument to a constructor or method in this class will cause a NullPointerException to be thrown").
Current String.format realization handles null for all conversions the same way, printing "null" (as specification says to do for "%s" and "%h" only).

Considering compatibility it would be appropriate to change specification and add the discription of this behaviour for all conversions. Or method implementation should be fixed.

String.format specification: http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/String.html#format(java.lang.String, java.lang.Object...)
Formatter specification:
http://docs.oracle.com/javase/1.5.0/docs/api/java/util/Formatter.html#syntax

Minimized test:
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public static void main(String[] args) {
        String[] formats = { "%c", "%d", "%o", "%x", "%e", "%f", "%g", "%tc" };
        for (String format : formats)
            System.out.print(String.format(format, null));
    }
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Output:
nullnullnullnullnullnullnullnull