I wonder if this is a game breaker.  Many corporate Exchange servers use a self-signed certificate, and push that certificate into client PC's root CA via group policy.

• With desktop PCs, you can push that certificate and install it
• With the iPhone, it seems that the phone can ignore this.
• With Windows Mobile 6.5, you can install additional certificates.
• But with Windows Phone 7, you can't install additional certificates, which means you can' synchronize with the Exchange server, unless you switch off HTTPS (bad - we're talking about synchronizing contact, emails over the air here…), or buy an expensive certificate from a trusted Root Certificate Authority.

Hopefully Microsoft fix this soon.