I'm using the newest binaries for kdenlive and mlt from sunab-svn on Kubuntu 12.10, x86-64.
When I stablize a clip which is 1080p I get a .mlt file for which kdenlive constantly creates a broken proxy: the proxy clip is framed by a big black frame. Creation of proxies for ordinary clips works as expected.

This is how the proxy clip looks like.