What does a scrum master do all day?

Please note: this question and answer is over twelve years old. The consensus understanding of the role of scrum master has moved on massively since then and so I no longer view this as a valid answer to the question, let alone one worthy of being the accepted answer. By all means downvote it. Beyond that, pay it no heed.

The Scrum Master will do things like ensuring scrums occur, organising sprint planning meetings, retrospectives etc. Also (s)he will be able to explain to management what the team is doing and why the team members cannot be poached off onto other projects until the sprint finishes. Beyond that, there aren't really any defined tasks for the Scrum Master. So one person should easily be able to be Scrum Master for 3 teams, and still have time left over to either do management type jobs (holiday requests, procedures, attending boring meetings with directors or whatever), or be free to contribute to the development resources of the team.

To make a long story short, the Scrum Master is responsible for making things happen. And in practice it is often the case that the Scrum Master is actually a project manager in disguise. At least that's the case in my company.

Working on this basis, and the fact that most businesses are running 2-3 projects at a time, what actual work tasks does a SM do to fill a full time job?

Anything within their skillset to help the Team achieve the goal.

Or, is it not a full time job and that individual do other things such as development, sales etc?

ScrumMaster was not originally intended to be a full time job. ScrumMaster is a role fulfilled by someone on the Team. That team member is dedicated to the product full time. So, when he\she is not doing ScrumMaster duties they default back to burning down tasks on the Sprint Backlog.