Only half joking...
Seriously though, one of the most powerful tools in consulting is a deep rolodex of unique talent. You never want to be in the position of telling a client you can't help them. You should always be honest, but you should also be prepared.
Someone could probably put together a pretty nice consulting association/firm with contacts made here on HN.
Seriously though, I wouldn't want to give up any of the flexibility I currently have, which includes both absolute veto over customers and projects, and "best effort" availability which ranges from "reply within 5 minutes because I happened to be checking my email" to "I'm busy with the baby today while my wife plays an orchestra gig" and in the extreme case "I'm tracking down a FreeBSD bug this week; hopefully I'll get around to helping you next week".
This may be incompatible with being contracted out by an agency.