More on the Plus Side
The new Can Spam Act contains some other potentially effective components. Specifically, it prohibits false or misleading transmission information and deceptive subject headings, and it requires a valid functioning return address or comparable mechanism to which recipients can respond. It also mandates that the sender of commercial e-email include information identifying the message as an advertisement or solicitation, as well as opt-out and physical addresses.
This means spammers can no longer use open relays without permission. It doesn't mean you should cancel your subscriptions to blacklist services that detect and prohibit incoming messages from open relays, but at least those services that target "legitimate" spammers will become more effective because it will be easier to identify the legitimate solicitors.
More on the Minus Side
The legislation may ultimately prove ineffective, because it puts even informed end users in a catch-22: It requires recipients to opt out of future mailings if they don't want to be contacted again, but how can you opt out if you don't answer? Suddenly, after training ourselves to simply delete unsolicited e-mail because spammers interpret even negative responses as positive, we're supposed to respond to every single message, opting in or opting out.