It looks like there was a patch done by Doug Ledford to add Pentium III support to the kernel. From what I could find, it looks like the patch was for different versions of the 2.2.* kernel, ie. 2.2.5, 2.2.12, 2.2.17 and later. Experimental support was added to the 2.3 kernel, and I believe that SSE support was fully added starting with the 2.4 kernel.