Keep on their system status for the next year...
I'm not going to claim the cloud is the silver bullet for all IT, but depending on 37signals investment in people, knowledge and hardware on networking; will be interesting to see if their system uptime stays similar to the cloud. There's a lot of gotchas in running your own data centre stuff. Scaling this stuff is hard.
If they have very well executed architecture and patterns for the common stuff (load-balancers, DNS, deploy tools etc), they'll be fine. If not, they'll be some humble pie to be eaten. Especially those here wanking over the data centre.