Portage checks the integrity of each file before unpacking any, among other things it is a security safeguard against anyone uploading tampered archives. For some reason, the MD% checksum is failing, despite it agreeing with the recorded one here. I suspect the file was corrupted when copied form the DVD to your hard disk. Try deleting /usr/portage/distfiles/linux-2.6.11.tar.bz2 and running emerge gentoo-sources again. That will force it to copy the file from the DVD again.
"Insanity: doing the same thing over and over again and expecting different results." (Albert Einstein)