If this article is about licensing then that's a task for a whole years worth of articles!

M$'s licensing is lame and according to their website on the availability of XP it's still available to OEM system builders until January of 2009 so till then you can still get your hands on OEM versions of XP. If you're a larger business who takes advantage of the Select License or Open License program you can DOWNGRADE your Vista license and install your Select / Open copy of XP till your hearts content.

OS's like XP will be at least supported for another 2 years since the service pack just came out. M$'s policy on support for service packs is generally 12 to 24 months according to their website.

http://www.microsoft.com/windows/lifecycle/servicepacks.mspx

I'd anticipate the majority of mid sized to large business stays clear of Vista and continues to use their XP media with downgraded Vista licenses.

Good luck out there...we're not upgrading ha ha!