SSL - Mobile Device Access Lock Down

We have a customer requirement to enable Direct Push email on our Outlook Web Access servers to a number of mobile devices the customer will be supplied from Vodafone - running Windows Mobile 5

Therefore we need to create a public HTTPS address to allow access to the OWA/OMA part.

We do NOT (at this stage) want to allow general access to OWA over HTTPS (we have an eGap solution with RSA for this) so we need to be able to lock down access to the OWA server only to specific devices. One way would be to use Firewall Rules at the Outer DMZ and lock down by the IP ranges of the phone but thats prohibitive to other devices and will fail when the phones change IP (i.e. international roaming)

Therefore Im wondering if we can use self signed SSL certs where there is no trusted CA provider (if there was all browsers would simply be prompted to trust the source and then get access). If we use our own self signed certs and have them installed on the client devices would this work? What would be the downsides (i.e. less cryptogrpahy without the CA part?)