According to this CNet News' report, Microsoft is considering to issue another major update to Windows XP that would follow Service Pack 2, already expected later this year.

[COLOR=#1951B9]Issuing an update to XP would represent a significant shift for the software maker, which for months has insisted that it had no plans to create a separate version of Windows before Longhorn. A company executive confirmed to CNET on Thursday that Microsoft is now discussing a product internally referred to as "Windows XP Reloaded."[/COLOR]

MS seems to be worried that the 4-5 year gap between XP's initial release and Longhorn arrival is way too much opportunity to give competitors such as Linux. If SP2 is a more security-oriented patch then a second update would most likely consist of additional and updated features currently available in the OS.