UPDATE
Microsoft pushed back a bit, with a message to ZDNet [& other sites] saying users should not install these updates to XP -- the linked article has been updated. To me their language is ambiguous. Microsoft does not say it won't work, or that it will cause problems. They want people & businesses to upgrade. They also want to protect themselves against lawsuits -- in the US at least you can be held responsible when you don't tell your customers Not to do something, e.g. there are product warnings not to use a microwave oven to dry your hair because of the risk of being sued, &/or maybe someone did sue for that very reason... usually companies settle lawsuits, paying however much on the condition everyone involved never speaks of it again to avoid bad publicity, so while you may not hear of a lawsuit, there is a very real reason for the warnings.
Updates for POS Ready 2009 will not include features that are in XP but not in POS Ready 2009, such as Media Player. If Microsoft didn't issue a warning, someone could probably sue after getting infected via a Media Player exploit that Microsoft knew about.
Microsoft's statement includes: "Windows XP customers also run a significant risk of functionality issues with their machines if they install these updates, as they are not tested against Windows XP. " I could buy that they weren't tested for XP machines using Microsoft Update -- I'm not sure that I buy into the idea that Microsoft created entirely different patches to provide the same updates to their paying customers, separate hotfixes that they did presumably check. They might have packaged the same updates differently, but personally I doubt they'd develop patches just for those machines if other Windows versions also needed the same patching -- Microsoft has always combined the fixes for as many Windows versions as possible in a single file, e.g. "Security Update for Microsoft .NET Framework 4 on Windows Server 2003, Vista, Windows 7, Server 2008 x86 (KB2931365)" which was included in the May 2014 updates.
And Microsoft may privately &/or internally want remaining XP users to install the updates -- if XP becomes a bigger target there will be more mal-ware & exploits targeting it, which will create more headaches supporting XP on systems where the owner pays Microsoft for continued support, e.g. the US gov, & banks with ATMs. Bill Gates was once quoted as saying he didn't mind pirated copies of Windows because that kept those users from installing Linux. That sort of thinking may still persist in Microsoft -- maybe they'd rather folks upgraded, but if they don't, they'd rather they continue to use XP rather than change to a Chromebook or Linux or a non-Windows tablet.
Whatever, we'll very likely never know -- Microsoft recently provided false info internally so they could catch the person(s) responsible for leaking internal data, plans etc., so the odds of someone leaking intel on this have just gone down. I would suggest a full Windows backup before adding the POS Ready 2009 updates, but then I'd have suggested the same thing before Microsoft ended XP support. Microsoft has erred before, I'm sure they'll error again, & that's totally separate from any POS Ready 2009 vs. XP issues or concerns.
That all said, here's some additional info that may help make up your own mind...
The differences between XP & the POS Ready 2009 versions are minimal... in a nutshell the POS Ready version is a stripped down copy of XP.
http://technet.microsoft.com/en-us/library/ee406170%28v=winembedded.0%29.aspx#Q3
This is allegedly the original forum thread where the registry mod was 1st posted.
http://www.msfn.org/board/topic/171814-posready-2009-updates-ported-to-windows-xp-sp3-enu/
Here's the article or blog that much of the press [e.g. ZDNet] used.
http://betanews.com/2014/05/26/how-to-continue-getting-free-security-updates-for-windows-xp-until-2019/
