You guys should have looked in the Mobile section of the forums. Several people have had their Pre's working with self-signed certs since the day it was released.
These are the instructions I used for my Windows Mobile devices and I imported the same cert into my Pre. I use EAS sync without any issues, but I have a buddy who is doing IMAP to Zimbra and he has no issues either. Don't go buy a cert because you have a Pre, there is no need.
1. Copy your [ZIMBRA INSTALLATION DIRECTORY]/ssl/zimbra/ca/ca.pem file to a safe location.
2. Run this command on the file you copied, it shouldn't change the original.pem file, but it's better to be safe than sorry!
openssl x509 -in ca.pem -inform PEM -out MyCertificate.crt.cer -outform DER
3. Copy the new MyCertificate.crt.cer file to your Windows Mobile device (Pre in this case).
Follow this Palm article for the remainder of the steps
Palm Support : Installing self-signed certificates
Release 6.0.2_GA_1912.UBUNTU8_64 UBUNTU8_64 NETWORK edition + Mobile Option
Activesync with Moto Q9C, HTC Touch Pro, Palm Pro, & Palm Pre