[quote] I can't believe anybody would be stupid enough to fall for that. [/quote]

It's not a question of belief; you've already got the proof.

Yeah, that was my question too. I think what happens is, when you enter your email address and password to login to the site, the site assumes its the same password you use to login to your mail provider. Of course, that assumption could be wrong but apparently, it works often enough. And of course, this only works with the big web mail providers like Google and Yahoo. I got a similar spate of emails from friends asking me to register on their birthday calendar.

Not really, they aren't assuming you use the same password as your mail provider. These sorts of sites just ASK you outright for your login/password.. and a lot of users actually give this up voluntarily to a third party site.

Lots of information here on this sort of anti-pattern:

http://microformats.org/wiki/social-network-anti-patterns

yep, it asks for your email login and pw - pretty common; facebook and twitter also do this (or did). people would rather allow them unfettered access than manually set up friends/followers.