Wanted to put this post out there in hopes of helping the unfortunate soul who may be trying to get this working as well.
Disclaimer: this solution has only been tested with the latest version of Nginx and OWA 2007. Your mileage may vary.
First off I would like to reference a very good example of getting Nginx set up with talking to OWA via a reverse proxy here: http://planetit.ws/linux/debian/configuring-nginx-based-reverse-proxy-for-owa.html
This example works perfectly for most mobile devices (Android, WebOS, etc). However, iPhones and iPads do NOT cooperate and simply fail their requests to the back-end.
After much searching apparently the problem is iOS devices do not work well (or at all) with GZip compression. To turn this off simply add the following to your nginx.conf and it will disable it upstream (the empty string disables):
proxy_set_header Accept-Encoding “”;
Once you restart Nginx with this added your iOS devices should work with ActiveSync with no issues.