I’m sorry to see that starting with version 3.3, the Horizon Client no longer supports Windows XP and support for the older version 3.2 client has only been promised through the remainder of 2015. I'm sure that a lot of existing customers have older Windows Embedded 2009 (still based on XP) thin clients that run an OS still supported by Microsoft through January 8, 2019. One of the major reasons to go to Horizon based VDI is to save on endpoint replacement costs and to extend the replacement lifecycle. Replacing hundreds (or thousands for larger organizations) of these thin clients with ones running newer versions of Windows Embedded or with Teradici based zero clients can be cost prohibative. If a core thin client OS is still supported by Microsoft, as a customer, I would expect VMware to at least provide a client to support it.
Unless VMware has a change of heart, later this year I'll be investigating solutions like Stratodesk, Wyse PC Extender, and Devon VDI Blaster that can essentially replace Windows Embedded (or XP) with a centrally managable version of Linux that runs the Horizon Client. However I'll have to do some testing to make sure performance is similar and that options like USB flash drive support and location based printing still functions as it does with a thin client running Windows Embedded. All of these products have a cost, but they are significantly cheaper than purchasing a new thin or zero client if it extends the usable life of them in a Horizon VDI environment for several more years.