I am not noticing many procs with Nightfall in general. I dont think it stacks directly, but "rolls" twice for each tick.
Corruption has 21.7% chance to proc Nightfall, while Drain Life has 18.5% (40%).
There should be a total of 80% chance to proc with the glyph, if you stack Drain Life and Corruption. But, I do not see it happen very often, which leads me to believe that it's not 80%, meaning the glyph and talent roll independently, or do not stack at all.
Blizzard usually indicates when something can not stack, so either it does, or they have yet to update the tool-tips.
Edited, Dec 12th 2008 4:36pm by sederix