Exchange 2003 should already have OWA/Mobile access set up.
Make your Exchange server a FrontEnd server. Install an SSL certificate on IIS where your Frontend server is. Update your domain record to something like mail.domainname.com to point to an IP address that you own. Create a route on your Firewall to allow that IP address to nat to your FrontEnd Exchange server via port 443 (ssl port). Make sure Exchange server is has SP 2.
That is basically it.
What you should really do tho is either get a ISA server or another Front End Exchange server into a DMZ which will then in turn connect to your back end exchange server.
Also note in exchange, under Global settings / Mobile Settings /Properties look at those options and configure to your preferance.
