When the backlinks module was introduced, it used the following syntax:
api.php?action=query&list=backlinks&titles=Foo
In July 2007, this was changed [1] to:
api.php?action=query&list=backlinks&bltitle=Foo
The first version was still supported for backwards compatibility, but
threw a warning saying support would be removed soon. The 'soon' part
wasn't exactly true (the warning was introduced 10 months ago), but it's
really gonna happen now: titles= support for backlinks, embeddedin and
imageusage has been discontinued as of r34518 [2]. This revision will go
live on Wikimedia wikis some time in the next few days, and will be part
of 1.13 and all later versions. Any client programs still relying on the
old behavior should've been fixed a long time ago.
Roan Kattouw (Catrope)
[1]
http://svn.wikimedia.org/viewvc/mediawiki?view=rev&revision=23774
[2]
http://svn.wikimedia.org/viewvc/mediawiki?view=rev&revision=34518