Padraic Brady has pormoted simplicity for unit testing in his latest blog post. He suggests that, for each test in your suite, you have only one assertion inside.

Obviously, a ratio of 1:1 would be perfect - but I'm sure we can accept some multiple assertions within a tolerance level. A ratio of more than 1.1:1 would indicate creeping obscurity (a signal it's time for me to intervene, refactor, and do some mutation testing for extra assurance). A ratio of 2:1 indicates a lost cause.

He notes that it probably sounds like a bad idea from the start, but he reinforces the suggestion with a bit of reasoning:

It's this notion of complexity that is often used to justify multiple assertions - if you break it down into the simple components that that complexity was built out of, multiple assertions don't have a leg to stand on (just the rare instances out of your control because they absolutely must be done).

You can see response that's been posted over on the PHP in Action blog in response to Padraic's thoughts.