The cut off date is June 30, 2008. According to the Wall Street Journal online:
Just to be clear: Businesses that are already using XP won’t be forced to upgrade to Vista. And a Microsoft spokeswoman tells us that businesses that already use XP will be able to buy additional copies for the foreseeable future. Some authorized dealers will also be selling XP for businesses until the end of 2008. Nonetheless, the cut off is definitely intended to start pushing businesses to Vista.
Frankly, I really like Vista, but some of our clients run vertical market applications that have only been capable of running on Vista for a few months. Since their businesses depend on these applications, reason enough to keep them on XP for the time being.
I’ll be blogging about some of my favorite Vista features in upcoming entries.