I''ve taken a quick look at Joyent's smart machines, and I can't find information to quickly answer this question. I'd like to know if they can be configured in a three-tier (or even two tier) architecture.
Is there a reasonable web-app-db server setup that they can be configured in? Ideally one where the machines talk to each other using an internal network, rather than billing for network use for their internal comm.
Yes, each SmartMachine has a public and private IP address so you can set up a multi-tier architecture. 10/40Gb makes it blazing fast too.
You can get the reference architectures here: http://www.joyentcloud.com/lpages/performance-and-scale/
Disclaimer: I am a Joyeur.