In the KB Article http://support.microsoft.com/kb/940881 MS talks about enabling the external autodiscover functionality for Outlook 2007 using a Service Record in DNS. This KB article is a little cryptic to me an therefore I'd like to write down the phrase one needs to add to a DNS servers zone table to make autodiscvocer work without the need of additionl external resources like SAN Certificate, multiple IP Adresses nor websites.
One simply needs to add the following line which represents a service record to the DNS zone of your company.tld Email Domain:
_autodiscover._tcp IN SRV 0 5 443 externalurl.company.tld.
From there on Outlook will detect the autodiscover setting using DNS and redirect the users to the externalurl.company.tld which in most SBS and EBS cases should be something like remote.company.com.