Whenever Microsoft releases a product, there is always a Support Lifecycle attached to it. There are several types of support, as can be seen looking at the Microsoft site here: http://support.microsoft.com/lifecycle!
How about Exchange? Below a table composed of information found at the same url (http://support.microsoft.com/lifecycle) :-)
Be careful with Service Packs! The following applies:
Support ends 12 months after the next service pack releases or at the end of the product's support lifecycle, whichever comes first. For more information, please see the service pack policy at http://support.microsoft.com/lifecycle/#ServicePackSupport .
Time for Exchange 2010 now :-)
Ilse Van Criekinge