Yes, this is true in theory, but I want to know how you're going to get VeriSign to issue you a certificate with subjectAltNames corresponding to a bunch of unrelated domains. And remember that ever time the ISP gets a new customer they have to get a new cert from VeriSign with yet another subjectAltName? This seems impractical.
If you are talking about TLS certs (not S/MIME certs) then the ISP can issue them to the customer directly (be a CA for connections from their customers over TLS connections). I have read that the customer can be given instructions on how to add the ISP cert as a trusted CA for that usage on M$ products.
I have no idea how to get M$ products to use that cert :-) as I do not use M$ products. I know how to do that on Unix.
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