The kernel 2.4 source code, the product of countless hours of coding, testing, and re-coding, by dogged kernel hackers, is being upgraded with patches and bug fixes.
Among the new kernel's features are: support for the IBM S/390, the Logical Volume Manager, NFS Version 3 servers, PCMCIA CardBus devices, USB peripherals, the Device File System, 64 GB of RAM (yes, that's memory, not disk space), Itanium and MMX processors, drivers for many additional hardware devices, greater maximum file sizes, and vastly improved Symmetric Multiprocessing capabilities, at least for certain processors.
The changes are more significant for non-i386 platforms, although developers on i386 platforms can make use of the improvements— and stumble over the incompatibilities—of the new kernel as well.
If there's a compelling reason to upgrade, the source code is available, in the tar archive: linux-2.4.x.tar.gz. (Refer to: "Where Is the Latest Kernel Version on the Internet?," below.) It will likely be several months at least before the commercial vendors upgrade their distributions to the new kernel.