Argh this is killin' me. My last email is wrong, I reread this again:
for example,
a warlock who takes Sneak of Shadows can use the
rogue's Sneak Attack class feature, which means that
he meets the prerequisite for the Backstabber feat.
There's the right answer, if it mentions the feature in the benefits, even if it is limited in use, it still qualifies you for the purposes of qualifying for feats.