Interesting. Thanks for the update. So the news is that they are merely toying with the idea of making $49/mo the new "cheap" plan. That would eliminate the free plan (do they still have that?) as well as the $12 and $24 plans.
The current signup urls are:
Maximum: https://signup.37signals.com/basecamp/Max/signup/new
Premium: https://signup.37signals.com/basecamp/Premium/signup/new
Plus: https://signup.37signals.com/basecamp/Plus/signup/new
Old Plans:
Basic: https://signup.37signals.com/basecamp/Basic/signup/new