The docs are updated to document this feature. Calling :Tabularize with arguments and no previous pattern prints an error message and does nothing. This works with both patterns and named commands. Suggested by nocash - http://github.com/godlygeek/tabular/pull/2
Per tpope: "I just discovered the second of the two tabular fixes I gave you doesn't work properly if the delimiter is at the very beginning of the line... [It] pretended the leading whitespace was part of the padding, which doesn't help if there's no leading whitespace." This reverts commit 16f7b1e.