There are serious rumors that Microsoft will release an interim upgrade to XP somewhere in 2005. This would fill the gap between Service Pack 2 and Longhorn, its next-generation version of Windows, scheduled by 2006. However, the stronger the rumours about a version to bridge the gap (and to generate revenue for Microsoft), the more analysts such as Gartner think that Longhorn may be later than expected. In the meantime, the possible interim version is dubbed "XP Reloaded".
Read more at An Interim XP For a Delayed Longhorn?
and at The Register (Even Microsoft can't wait for Longhorn)