The libssh2 version in squeeze is currently a little out of date.
The API changes are stable (none of the existing symbols were changed, only new ones added) so using libssh2 1.4.2 on squeeze should not cause any issues for other programs/libraries.
In particular of interest to myself is libssh2_session_handshake() added in 1.2.8 and needed for one of my projects which needs to run on squeeze. This function starts a SSH connection on an already connected file descriptor.
I would appreciate it if anyone could review the backport and upload it into squeeze-backports.