Oh cool :-)
I just noticed that selenium IDE has the option to export tests as PHP unit tests.
This is great - the IDE is a good way to interactively build up tests by recording a set of actions and tweaking what it records.
And being able to save them as unit tests makes it easy to integrate this with automated testing.
Interestingly they are created as PHPunit tests - not simpletest tests.
It does look like I'm being pushed more towards PHPunit - though I wonder how I'll integrate this with Drupal development and it's focus on simpletest.