[texhax] Strange interaction with revtex

[Michael Barr]

I got a query from someone trying to use my diagram macros in conjunction 
with revtex.  Finally, I decided to try to track it down.  What follows is 
really strange and I could not begin to imagine how it could be done. 
Here is a sample file:

\documentclass{revtex4}
\newdimen\ul
\ul=.01em
\showthe\ul
\begin{document}
\end{document}

Try it; you get the message that \ul=0pt.  If you put it after the 
\begin(document} you get the correct value.  How can this be done?  Lest 
you answer that I should not be using \newdimen, I point out that the 
diagram macros work fine with plain tex (and anything else I have tried 
them with).  Of course, \ul is a basic dimension in them.

Can anyone explain what is going on?

Update: I tried changing \ul to \UL in the above file and the results were 
the same.  If I changed em to ex, same result.

Michael