Click here for a copy of the law. Read it through, and make yourself familiar with it.
If you send mail, opt-in or not, especially if you think you send opt-in mail but some places block/filter/reject it, keep this in mind: CAN-SPAM does not give you legal recourse to stop sites from blocking your mail. If you think an ISP has to accept your mail because you comply with CAN-SPAM, then you didn't read section 8:
SEC. 8. EFFECT ON OTHER LAWS.In other words, just because your mail is legal doesn't mean ISPs are forced to accept it. I interpret section 8(c) generally to mean that ISPs are allowed to set policies with regard to what mail they'll accept or transmit. If you're a mailer, that means ISPs can block you if they so desire, based on whatever policy they have in place.
(c) NO EFFECT ON POLICIES OF PROVIDERS OF INTERNET ACCESS SERVICE-
Nothing in this Act shall be construed to have any effect on the lawfulness or unlawfulness, under any other provision of law, of the adoption, implementation, or enforcement by a provider of Internet access service of a policy of declining to transmit, route, relay, handle, or store certain types of electronic mail messages.
If you're an ISP and are concerned about being able to terminate a spamming client even though their mail complies with CAN-SPAM, this is easy to address. Make sure you have a contract that forbids bulk unsolicited mailings. Don't use the word "spam" or you get stuck in an argument about what spam is or isn't. Make it clear that if you have proof that they're sending mail to recipients when they don't have direct, verifiable consent to mail those recipients, that you are able to break the contract. This would be your policy of declining to transmit certain types of electronic mail messages from your network.
I'm not a lawyer. If you think the law is going to affect you, go get a lawyer and have them review the law with you. Beware the marketing consultants spreading FUD (Fear, Uncertainty, Doubt) about the new law. A lot of it is questionable advice. Keep in mind that if you're a mailer already doing things right, the impact on you is going to be minimal.