Apologies for not getting back to you sooner. It sounds like you already have OWA (Outlook Web Access) configured. If you head over to Microsoft.com, and do a search for "activesync configuration", that'll probably point you in the right direction. Getting your SSL certificate loaded on your Treo is a must, though.
I haven't used that specific device to do this before, but I have done it with other Windows Mobile devices (I use a Starcom PPC6700 myself), and it works more or less flawlessly. The only time I use the VPN from my device is when I want to VNC into a server in order to restart it remotely.