Some time ago I used kernel-2.5.47/48 without any troubles, but when kde3.1_rc3 came out i couldn't emerge kdelibs because asound.h and another header file (afrequenzer.h if i'm not mistaking) couldn't be found. the first hit on asound.h on google was alsa. so i figured out it was a alsa sound header file that was required by kdelibs to compile against (or with) but because in kernel 2.5 alsa will be in the kernel included. I think the files are on other locations or other names or they don't even excist. when i took a step back to the stable kernels i had no problems. so it must be the 2.5 kernels. I don't know if this is a ebuild/portage bug or KDE, but i do know that kde isn't 2.5 ready. I don't have the error log anymore... maybe i can reproduce it on a test machine. sorry that i didn't submit it as a bug earlier :/
I can't reproduce this failure on a clean system. but if I want alsa support in kde I give the alsa useflag and emerge wants me to emerge alsa-driver and libs what fails because alsa is already in the kernel. I think the reason for not able to reproduce this failure is that i used vanilla-sources first and compiled kde-3.0.4/kde-3.1rc1/kde-3.0.1rc2 with that one (with alsa support) after that i used the development sources and upgraded to kde-3.1rc3 ..
Just emerge inject (cf. emerge help) alsa-driver (and alsa-lib if the 2.5 kernel provides that too?). That will make emerge behave as if these packages were emerged and deps on them were satisfied. The 2.5 kernel ought to provide asound.h still, since it's the most important/central alsa 0.9x header. Then again i haven't tried it yet myself, so can't vouch for it...
In fact, the latest alsa-lib does not depend on alsa-driver any longer, so it should not be neccessary to inject the alsa-driver package.
OK, then there's nothing to fix here, regardless of what the 2.5 kernel does.
