Using the following plain TeX input
just a plain line of text in the middle of the page
The line claims it is in the middle of the page, but if I \input tikz
it is not. If I don't \input tikz, the line is (more or less) in the
middle of the page, as expected. Also, when I \input tikz, my page
number goes away.
This is the version of tikz in the texlive 2010 as of Nov 6 (pgf 2.10)
The previous version of pgf/tikz in texlive did not exhibit this misbehaviour.