No, they are not, They wouldn't contribute anything new to master anyway would they?
You can check yourself
git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
git remote add stable git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git
git fetch stable
gitk -n 50000 stable/linux-3.11.y stable/linux-3.10.y master
Navigating single commits is too tedious, but using the "Follows" hyperlinks you can get reasonably easily down to e.g. 3.10 and then using "Precedes" up the 3.10.12 again (For 3.11 it's and its stable branch it's even shorter). You'll see the that stable branches involve no merging in either direction.
Should you get lost use history buttons (arrows) right of the SHA1 field to get back where you came from.