Paul Thurott has written up an FAQ on the last service pack that will be released for Windows XP. A few of the basics:

• Indeed, SP3 will be the last service pack for XP.
• It can be installed even if you hadn’t installed prior service packs.
• It took this long to release SP3 mainly because Vista was on everybody’s mind.
• New features: Network Access Protection (NAP) support, product key-less install option, a new kernel module for several different cryptographic algorithms, “black hole router” detection (router that incorrectly reports their network path information).
• Release date of Windows XP SP3: second quarter of 2008, or three months after the release of Windows Vista SP1 and Windows Server 2008.

And as Paul put it:

Nothing else new. There are updated applications, which shipped long ago as separate downloads, like Internet Explorer 7 and Windows Media Player 11. And there are even some features that have been removed, like the taskbar-based Address Bar option.

Check out the FAQ for more info. Then after SP3, enjoy what you have with XP because that will be it.