I just upgraded to 2.6.24-r4 and everything seems to be OK. However, when I tried to emerge dev-util/systemtap-0.7_p20080405, it failed with systemtap requires support for KProbes Instrumentation (KPROBES) - this can be enabled in 'Instrumentation Support -> Kprobes'. Then I looked at the kernel config file /usr/src/linux/.config and the KPROBES option is no longer present. It was there in 2.6.24-r3. I'm going to attempt a re-emerge of 2.6.24-r4 to see if that fixes it. "emerge --info" coming.
Created attachment 148981 [details] emerge --info
A little more research ... I found bug 215442 by reading the comments in the re-emerge, so I tried that and it seems to have put KPROBE back. However, "genkernel --menuconfig all" is picking up its config file from /usr/share/genkernel/x86_64/kernel-config-2.6, and KPROBE is gone again. Maybe this is really a genkernel (Gentoo Linux Genkernel; Version 3.4.10_pre7) issue??
That bug should be fixed with gentoo-sources-2.6.24-r7 and gentoo-sources-2.6.25-r2.