In the UK, you should use opt-in by default. You can only use opt-out to send unsolicited mail in specific circumstances. From the ICO's "Rules on email marketing" PDF:
Opt-out is where you are told that you
will get marketing unless you say you
don’t want them. Organisations can
collect only your email address on an
opt-out basis if they can satisfy the
exemption criteria:
- your email address was collected ‘in the course of a sale or
negotiations for a sale’;
- the sender only sends promotional messages relating to their ‘similar
products and services’; and
- when your address was collected, you were given the opportunity to opt
out (free of charge except for the
cost of transmission) which you didn’t
take. The opportunity to opt-out must
be given with every subsequent
message.
Even if you are eligible to collect email addresses with an opt-out system (i.e. you're selling a product, marketing only similar products, and offer a clear way of opting-out), it's usually a bad idea, because you end up with people on your list who don't remember subscribing and who will report your email as spam. From the MailChimp blog:
Opt-out: This is an old-fashioned way
of building your email list where
you'd typically have some form for
people to fill out (like to receive a
free whitepaper or something). Hidden
at the bottom of the page would be a
little pre-checked box, with something
like, "Yes, please sign me up for your
email newsletter!" It's sort of a
scummy way of doing it, but
technically it's legal. We highly
recommend against it, because you'll
end up with tons of people who don't
understand how they got on your list,
who won't read your emails, and who
will send complaints to the anti-spam
authorities to get your server
blacklisted. It's yucky, so stay away
from it.
Lists are only valuable when they contain subscribers who genuinely want to hear from you. There's little point in marketing and communicating with people who couldn't care less.
As such, double opt-in, where users first tick to receive information, then click a link in their email to confirm their address, is the best way to build a list. If people are prepared to do that, the chances are high that they'll be prepared to open and click links in future emails you send them, and that's exactly the sort of customer you want on your list.