The usual solution to the problem of installing CSSU is just running apt-get update and apt-get upgrade. On rare occasions, such as now, there is a problem when something disastrous is in the pike to be upgraded. I presume that using the hold code recommended earlier will solve that problem, so update and upgrade should fix you up.

I have had my devel and testing repositories enabled since I got my N800, then N810, then N900, without major problems. I have been running apt-get update and apt-get upgrade more than once a day most days.