On: Your Wireless Router Is Broken – Help Us Fix It At...

[hrrsn]

Can easily be obtained by spear phishing or nmapping IP ranges.

[zdw → hrrsn]

Or frequently by the email header of someone who sent mail from behind the router.

[Kayou → hrrsn]

Do you need to be on the same local network as the user to do that or does that work with an attacker being on the internet?

[hrrsn → Kayou]

If you just need their external IP address, you can probably easily coerce that out of them by getting them to click a link. Send an IM to a bit.ly link that logs an IP and forwards on to some random image, an email, a tweet, etc.