Hey Neenz, good thread - and timely, too.
There hasn't been much news on the CC front since
our last major ding dong in Spring 2007 (see here
and here for the two threads if you have time on your hands).
To answer your question:
No, there are no studies measuring the long-term impact.
There are studies looking into the short-term impact with the
help of parent questionnaires and none of them find anything untoward.
Have a look at this fairly comprehensive review of all the pro/con research
out there here - a mother had asked a Google researcher a similar question to yours
and his verdict (I paraphrase):
'The anti-CC lot might have a point but they are not making it.'
They cannot make their point as they have no evidence. You either believe in it or not.
The anti-cc equivalent to that page is this blog. Just about anything that supports
the blogger's point of view has been linked here.
I have worked my way through every single document - and some of them have been quite interesting -
but not a shred of evidence that CC does damage, short-, medium- or long-term.
Right, more after breakfast.
Btw, this has been one of the most civilised, yet interesting sleep training threads for a while.