*Updated to account for Stopped (Deallocated) which means stopped from the portal, not just shutdown from inside the instance!!!
You asked for it Microsoft did it. Billing per minute is pretty self-explanatory, prior to June 3 all IaaS compute hours billed by the hour and for the most part rounded up, no more, now you are billed per minute!
However the best thing related to billing was the Stopped (Deallocated) VMs no longer incur compute charges. This is very important for Dev/Test environments. Many customers were exporting their VMs to a blob store nightly and over weekends to stop the charge per hour for a test machine. Now you can stop the compute clock by stopping (Deallocating) the VM. If you only have your VM turned on for a few hours a day you will only pay for a few hours a day. Keep in mind, you will still be charged for other resources that may be part of your deployment, for instance if you have 16 disks attached all at 50% capacity, you will continue to get billed for that as you are still occupying space. This only applies to compute hours, I now this will make a lot of developers and testers very happy!
Windows Azure: Announcing New Dev/Test Offering, BizTalk Services, SSL Support with Web Sites, AD Improvements, Per Minute Billing