I've installed slim in place of gdm on my smatbook. But there is a problem with plymouth (i think) and the five square (under the genesi logo) continue to blink.
These square "LED" appear always on top of windows after login (tried with awesome and openbox WM).
I was able to get the same problem you did. The problem is that the slim display manager has not been converted to an upstart job so no events are generated that cause plymouth to stop like they would with other window managers like xdm or kdm. the easiest way to fix this that I have found is to simply edit /etc/init.d/slim and add the command plymouth quit before it starts the display manager inside of the start portion of the case statement.
Hope this helps. if its not clear I can paste the relevant portion of the init script with my changes in bold text.