Since the cert is self signed I'm not sure you can properly install it on the device (I could be wrong on this though).
I'm assuming since you're using a self-signed certificate you are the exchange server admin and the server isn't really used in production (as expecting the end user to get around that would be a nightmare). Although my method can be used with end users if your deployment is small enough (mine was when I used this method).
The easiest thing for you to do would be to get a proper certificate from
www.cacert.org. Their signing service is free which is awesome and they support "UCC"/Subject Alternative Name certs as used by Exchange 2007. The only caveat to using CACert is their root certificate is not installed by default in Windows Mobile. Install that on your device after installing the certificate on the server (there is a root cert link on their homepage) and you should be golden.
If your setup is big enough to need a "real" cert the cheapest one I've found that does support UCC certs is from GoDaddy. I was able to snatch a good three year cert for $50 a year and end the cert install on devices manually on each hard reset.
If you need any help with the admin portion (getting the cert on the server or device) chime in. Otherwise, let me know it worked out well
