Hi Jeff,
The first thing I thought when I read your post was "Why only for some services?" - I would recommend that you implement SSL for both the portal and backend systems end-to-end, I can't see any good reason why you wouldn't want to do this. SSL overhead is relatively low these days (with modern CPUs etc...) I don't think you need to worry about it. It also avoids any secure to insecure warning messages you might get if switching between both.
So what you've done is technically OK and will work, but I would step back and question is it the right decision, to my mind it's not.
Hth,
Simon
