It wasn't a secret, but the developers have made a formal announcement
The new Ubuntu 13.04 (Raring Ringtail) is knocking at our door, which prompted the Canonical developers to announce what Linux kernel will be used, officially.It's not a secret that Ubuntu 13.04 (Raring Ringtail) was going to use Linux kernel 3.8, especially because the latest (and only) Beta release was already based on this version. Nonetheless, it's nice to have something official.
“Now that we are past the Ubuntu Raring Ringtail 13.04 Kernel Freeze, the Ubuntu Kernel Team felt this would be an appropriate time to officially confirm the Ubuntu 13.04 kernel will be based on the upstream v3.8 Linux kernel,” reads the official announcement on the Ubuntu devel mailing list.
Some of you might wonder what's changed, with the new kernel now firmly in place. Canonical has provided a wiki page detailing all the difference between Ubuntu 12.10 and Ubuntu 13.04.