ISA in SBS - yes, it's secure
Not strictly an ISA issue but you might be inclined to think so when troubleshooting, so let me save you a little bit of time. When you install SBS2003 SP1 a new security certificate is created. Once you've created your new certificate, users of Outlook over the Internet must accept the new certificate. Simply clicking YES on the certificate prompt isn't good enough for HTTP over RPC.
The quickest way to get the new certificate is to have the user attempt a connection to OWA. This will generate a certificate prompt from which they can install the new certificate to their computer.
To add the certificate to you computer, click View Certificate on the certificate prompt. Then click the Install Certificate button.
No need to go any further, the user can now close the browser and use Outlook over the Internet.