It took me a while to get back to this.
While I normally would not object to any ISP using ... content filters, the "get another provider if you object" argument doesn't work nearly as well if there's only one DSL or cable provider that serves the customer.
Without that argument, there's a clear path to "since your customers have no choice, you are not allowed to filter content." While this probably applies to DSL since it's "like telco" and there's already legislation about what telcos can't filter because of their old "natural monopoly" status, I don't think it applies to Critial Path or MSN or AOL or any other mail server operator -- there is choice, and thankfully, there is no hint of legislation coming for "mail server content filtering policies."