#!/bin/sh
man -t $1 > /tmp/$1.ps
open /tmp/$1.ps
This will create a PostScript file for the command you're trying to get the man page of, store it in the /tmp directory, and then open it using Preview.
[robg adds: Though this is technically an update to an older hint, I'm running it as new because (a) it's much simpler than the previous solution, as ghostscript no longer needs to be installed, and (b) because it's so nice. I created the script and called it preman and made it executable (chmod 755 preman). Now it's a joy to browse and read huge things like the tcsh man page. There are, of course, third-party apps like ManOpen and manThor that do the same thing, if you prefer a pure GUI solution.]

