We can see with 
http://code.google.com/p/txt2tags/source/browse/trunk/test/art/toc-macro.t2t
that try to have a title for a TOC with a level 1 title is a dead end, because 
when you don't use the --toc option, the TOC doesn't appear, but its level 1 
title is still here :
http://code.google.com/p/txt2tags/source/browse/trunk/test/art/ok/no-toc-macro.a
rt
http://code.google.com/p/txt2tags/source/browse/trunk/test/art/ok/no-toc-macro-s
lides.art

Conclusion : we need to bind the TOC and its title.

Proposition : a title parameter for the %%TOC macro : %%TOC(Table of Contents)