When used in math mode, it is a binary operator in both LaTeX and plain TeX.
fontdef.dtx:868:\DeclareMathSymbol{\dagger}{\mathbin}{symbols}{"79}(As far as I know, LaTeX does not change the math classes of any symbols from the plain definition.) Of course it is possible to use \textdagger to typeset the same character in text mode, but that's unrelated to math mode. -k
