I signed up for the Windows Azure Platform Introductory Special. You get 25 hours of a small compute instance per month which is more than enough for my current development needs. I checked the billing after a couple of days and was surprised to see that I had used 4 hours of compute time. I'd only deployed a test application and loaded some test data. The compute instance had also been suspended for some of that time as well.
The little light in my head switched on.... COMPUTE HOURS equals DEPLOYED HOURS, suspended or otherwise. The only way to stop the clock ticking is to delete your application.
I still think 25 hours per month is good enough for my needs and the Windows Azure Platform Introductory Special is a great way to get used to Windows Azure as a commercial platform - just watch those hours.