Truly the easiest way to get external clients connected to your SBS 2008 is a third party trusted cert. But what kind? The expensive SAN cert? The wildcard one?
Here are the ones that work –
Godaddy $20 special. Yes the cheapest cert Godaddy has will work with SBS 2008′s Outlook Anywhere/Exchange and all that because of the magic from the SBS dev folks. One caveat is that this low priced cert does come with that funky intermediary cert that your cell phones either need to get that intermediary on the device or they need to natively trust Godaddy from the get go (open up the phone, go to the trusted certs see if Godaddy is listed)
If you do not use the trusted cert, remember that you need to provide a way for that self signed cert to get deployed. I’ve stuck it up in SharePoint for folks to pull down if they are not on the internal network during the beta.
The Official SBS Blog : How Do I Distribute the SBS 2008 Self-Signed SSL Certificate to My Users?: