M.S.
(I haven't read all of the previous replies to see if this has been mentioned or not.)
Everything I've read says that 9-12mos is the worst time to introduce 'sleep training', because of the super light rem sleep (due to large motor skills being practiced in their sleep). Last I read Ferber and his comments on crying-it-out, he said he doesn't even recommend his method until after 12mos of age.
This may be something that you have to ride out, being consistent/loving/responsive.
I've also heard this happens again between 15-18mos, when the second wave of separation anxiety kicks in.
Good luck!!!!