If you are in a game, and it is saying "diablo walks the earth" he will be the first unique monster you come accross. Eldrich or Rakinishu are easy ones to find, if it's a public game chances are someone spawned him away from where you would normally look. Here is a list of unique monsters any of which *should* spawn uber diablo. I haven't tested this but im not sure act bosses will be replaced with uber diablo could be possible but I dunno. Also he will ONLY spawn in Hell difficulty, wether or not you still see the messages when diablo walks in the other difficultys is a good question.
http://www.battle.net/diablo2exp/monsters/super.shtml
Edit not that you're probably in the game still but hey next time you'll know!
