"package upgrade behavior" in Synaptic
 

I found out that if you are going to use the Debian Lenny backports repository in Synaptic, you must set Synaptic to prefer that repository. Go to preferences-->distribution tab, and set "prefer versions from" to lenny-backports. Without this, my Synaptic wouldn't even display the versions offered by lenny-backports.

yes, with aptitude you have to specify backports.

aptitude -t lenny-backports install foo