Thanks to everyone for the advice.  My resolution went in a completely
different direction though.  Thanks to some advice from Adam M I learned
that the MS VPN client is just a PPTP client and doesn't require ISA at
all.  So I ditched the ISA box (I'm sure there's some cheering out there
somewhere) and just enabled PPTP passthrough on my router.  My trusty 678
now sits in bridged mode and my router does the PPPoE (no worries about it
starting back up after power failure and such as with ISA).  With built in
masquerading on the router and PPTP passthrough to a box behind it for VPN
auth I'm set.  Much simpler than ISA and does everything I wanted it to.

Again, thanks to everyone that offered options.
Ben.
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