I'm surprised that kernel 3.12 works for you. I tried to boot R2rc1 1 week ago and the only kernel that booted was 3.9. And I have the exact same laptop : T410 with intel graphics and i5 M520. The only thing I'm not sure is the BIOS version.
BTW I noticed that even if I installed with 3.9, grub always boot from 3.12 by default and I think with beta 3 it kept 3.9 as default.