[texhax] Upright digits in theorems

[Alex Scorpan]

What is the most elegant and robust way to arrange that digits and  
such always display upright inside a theorem-like environment, when  
the surrounding text is italic?  This is for a rather large-scale  
application.

Our current approach is to make the various characters active inside  
the theorem environment and define them as \textup{...}.  However,  
that creates problems if anybody uses, say, a label with a digit or a  
\vskip or whatever.

-- Alex