Chris, many thanks for your enthusiasm and willingness to help with Moodle 2.0 testing. (I'm still fast asleep at 5.30am!)
Tim's reply is spot on as usual (thanks Tim!) and I've just created a page explaining the process: QA testing.
However, before you start assigning lots of issues to yourself(!), please wait until I post officially about QA testing (hopefully next Friday). The reason for delaying QA testing just a little longer is because the last of the big changes are due to be added to CVS this week - enrolment interfaces, assignment interfaces, messaging interfaces and restore, according to Martin.
Otherwise, please continue testing any of the things mentioned in the testing focus in the latest Moodle 2.0 preview notes (3 nearly 4, as you kindly pointed out; forum introduction updated accordingly ).