I guess they shouldn't have been making promises like this then, under the headline "Now and Forever".

When a customer signs up through your referrer link, you’ll earn 50% of their first month’s payment (after they make that first payment), then 5% of their future monthly payments for the life of their account.

http://37signals.com/svn/posts/1063-launch-announcing-the-ne...

(Google cache version of same): http://webcache.googleusercontent.com/search?q=cache:CYZy2_t...

I'm sure that there's a lawyer-written T&C doc somewhere that explains how any claims in that post are a lie and they own your firstborn, but this seems shady.