Besides the extra functionality, the 257.21 release includes numerous bug fixes as well as significant performance improvements on a number of game titles. You can find out more about these in the official release notes, which are available separately for Windows 7 or Vista and Windows XP (in PDF format).
The new 257.21 drivers are also available for laptop users running either Windows 7 or Vista -- in 32-bit and 64-bit flavors -- but not Windows XP. Highlights are pretty much the same as in the desktop but also include support for Nvidia Optimus graphics switching. You can download the latest WHQL release by choosing the appropriate version on this page.