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Customers can now choose when their applications receive new operating system updates and patches by selecting which version of the operating system their applications will run on in Windows Azure. Right now there is only one available operating system version (released on December 17th, 2009), but new builds with the latest updates and patches will be released regularly. This new feature allows developers to test their applications when new patches come out before upgrading their production deployments.
To select an operating system version for your application, add the new osVersion attribute to your service configuration file. The full list of available operating system versions is maintained in the Configuring Operating System Versions topic in the Windows Azure MSDN documentation.
Does Microsoft reserve the right to apply an update if it represents a critical security vulnerability, or will the customer retain complete control over which OS can be applied.
Also, is there any information pertaining to how many back versions of the OS that will be maintained, and for how long a customer will be permitted to remain on an older version.
You may want to post your question to this forum: http://social.msdn.microsoft.com/Forums/en-US/windowsazure/threads
You should get a response and some guidance here.