After installing the plugin-manager (on a fresh debian lenny system), getting "ftp server not found" errors, reading through numerous bug reports here, I installed the libssh2-php debian package (from backports).

After that I got the option to use SSH when installing a module with the Plugin Manager.
But using this produces numerous lines of this error message:

warning: ssh2_scp_send() [function.ssh2-scp-send]: Failure creating remote file in /usr/share/drupal6/modules/plugin_manager/ssh.backend.inc on line 78.

What's wrong?