Just a bit more than a week has passed since the release of the Linux 2.6.31-rc2 kernel, but Linus Torvalds has today replaced that with the third release candidate in the Linux 2.6.31 series. The Linux 2.6.31-rc3 kernel drops the newly-introduced Langwell USB OTG driver since this new code isn't yet ready for mainline support and thus it has been reverted for this series, which is actually the single biggest change in this new release candidate. The Linux 2.6.31-rc3 kernel also brings various driver fixes, graphics updates, architecture updates, and more. The Linux 2.6.31-rc3 kernel release announcement can be read at LKML.org...
I think a more interesting metric would be how much the average memory footprint of the kernel and its loadable modules has changed over time.
Although the overall size kernel source has grown, that size isn't necessarily a reflection of what makes it to a running kernel. For example I only load a few of the file systems available and certainly only load a few of the hardware drivers available. While other processor architectures are supported, I only load one.
Although the source size is still interesting as a software engineering statistic and a metric of collaborative work, it's less interesting for measuring the practical reality than one might think.