You probably want this specification in overo.inc file (meta-gumstix/conf/machine/).
Fetching failure can be a result of busy server. I've seen it happen a number of times. Of ~3000 required packages, any number of package sources could be down at any moment. If Steve's server is slow, you can modify the kernel 3.5 recipe to point at git://github.com/gumstix/linux.git
. I have been trying to push an updated meta-gumstix layer (number of changes including kernel source hosted at Github), but haven't had the chance yet.