Apple has recently released Mac OS X 10.4.2 for the x86 platform (actually, it’s only provided to developers as a Developer Transistion Kit, DTK). One of the changes is that the 10.4.2 development tools now generate application binaries with new header information (Mach-O file header). This new header is incompatible with 10.4.1, i.e. on your 10.4.1 x86 system, you cannot run software which was compiled on and for 10.4.2.
This patch modifies the 10.4.1 kernel to adapt for these changes. This means, you should be able to run 10.4.2 software.
Please note, that new software which is built for 10.4.2 might require up-to-date versions of specific system files, e.g. frameworks or libraries. In this case, you would also need to update these files. The software I tested so far, however, do not require newer system files.
As a side note on the patched kernel: It also contains the v0.5c SSE3->SSE2 emulator and runtime patcher, so that you can run Rosetta and 10.4.2 software. (Users with a SSE3 CPU: see also FAQ.)