I have had mana regen drop dramatically twice over the past couple of weeks. However, there were two very good reasons for this. Before you cry nerf, make sure:
-Your armor has not gotten to 0% (3k mana vs 6k mana makes Lich a lot worse!)
-Your archlich has not accidentally been dispelled (due to low health/debuffs/etc)
Aside from these two instances, I haven't seen any change in mana regen...
GS