I thought Microsoft has covered the life cycles for their products well enough but questions about it keep coming. A recent thread in the newsgroup is a good example.
Very details answer to this was given out by the Product Team in their blog - XP Embedded Lifecycle Support.
Some more helpful links on the subject.
Life cycle of Service Packs for Microsoft Embedded products is detailed here.
Here is what it says about Windows Embedded:
Product Name General Availability Date Service Pack Support Retired Notes
Windows XP Embedded Service Pack 0 (RTM) 30-Jan-2002 22-Oct-2004
Windows XP Embedded Service Pack 1 22-Oct-2002 10-Apr-2007
Windows XP Embedded Service Pack 2 18-Jan-2005 Not Applicable
See Note Support ends either 12 months after the next service pack releases or at the end of the product's support lifecycle, whichever comes first. Visit the Lifecycle page to find the support timelines for your particular product.
Microsoft Support Lifecycle page for Windows XP Embedded.
According to Shayna Begun [MS], Feature Pack is not the same as a Service Pack - Feature Pack 2007 did not reset the support lifecyle for XPe. Regarding the next version of the product, it would be the next version of the Embedded product, so Vista would not pertain here.
Andy Allred [MS] confirmed that as well here. (you can find the whole thread here)
Just thought I'd gather all the related info in one place.