Pros/cons to consider in a few contexts: * Custom domains - if you ever want to allow custom domains (ala Tumblr, Posterous, etc), it's probably easier to go w

Pros/cons to consider in a few contexts: * Custom domains - if you ever want to allow custom domains (ala Tumblr, Posterous, etc), it's probably easier to go w/ subdomains. The URL recognition will be much easier. * Security - if you ever want to allow custom HTML/Javascript, I would say that custom domains are a must for security reasons. However, passing login credentials across multiple domains might be trickier than using a single domain. * SEO - I'm not completely sure, but very curious to know. Can someone chime in on SEO tradeoffs of these two options?