Ok - so I have a new SBS Premium installation and all is going peachy, except that I can't get Outlook over HTTP working.  It's really annoying me, as I haven't had any problem with any other SBS installation - Outlook over HTTP works great for all of them.  Mind you, OWA & RWW are working great - so I know 443 is getting to the external nic on the SBS ok.  I reran the CEICW and verified that 'Outlook over the Internet' was selected in the firewall configuration section.  Just to be safe, I compared the ISA settings to those on a working SBS - and everything matched - destination set, web publishing rule, etc.  I even checked the permissions on the /rpc directory under the default web site in IIS - everything is set just like it is supposed to be. 

So I started digging through my ISA logs, and in the web log I found where my connection attempts were hitting the server, and being dropped.  The log revealed a status code of 502, which according to Google indicated a 'Bad Gateway' error.   Huh?

So I posted to the Partner Newsgroups (you guys are aware of the MS Partner Newsgroups, right?  www.microsoft.com/partner )  Outlining the problem.  Woody Guo pointed me to URLScan, which was indeed installed on this machine as part of ISA FP1.  Specifically, he indicated the following edits were needed to the urlscan.ini file:

[RequestLimits]
; The entries in this section impose limits on the length
; of allowed parts of requests reaching the server.
MaxAllowedContentLength=2000000000
MaxUrl=16384
MaxQueryString=4096

In addition, you need to add the following verbs to the Allow Verbs:

RPC_IN_DATA
RPC_OUT_DATA

After editing the ini file, restart IIS Admin Service and Microsoft ISA Server Control services.

Needless to say, this did the trick and the client's Outlook over HTTP is working like a charm!