Firewall problems with HTTP/Apache2 access to repository?

I've set up mod_dav_svn on Apache2+Linux.

In general it works fine, except for clients that are within a company with
a pretty strict firewall. When those guys try to access the repostiory,
they get this experience:
homeslice_at_myhouse:~$ svn co http://someserver.com/svn/moos-ivp/trunk/
svn: PROPFIND request failed on '/svn/moos-ivp/trunk'
svn: PROPFIND of '/svn/moos-ivp/trunk': 405 Method Not Allowed
(http://someserver.com)

On the server side, we see this show up in Apache's access.log file:

<<Some IP #>> - - [01/Nov/2007:11:41:49 -0400] "PROPFIND /svn/moos-ivp/trunk
HTTP/1.1" 405 371 "-" "SVN/1.4.4 (r25188) neon/0.26.3"

Does anyone know why this might be happening?

Thanks,
Christian