This indicates that folks are selling their SOJs (mostly duped, surely) to try and "pop" the Uber-Diablo. (check out those stats, baby! http://www.battle.net/diablo2exp/monsters/act5-uberdiablo.shtml) There's a thread about this just a few below yours and tons of information on him scattered around if you do a search on world event or Uber-Diablo.
try searching for "diablo clone" on these forums. that will give you thousands of threads with info of what/when/how/why etc
To explain it a little brief just to tease your interest...
Basically, its a counter. When that counter of sold sojs hit a specific number (nobody knows which), the diablo clone will spawn. If you manage to kill him, he will drop the unique small charm "the annihilus", which has the following stats:
+1 to all skills
+(10-20) to all resistances
+(10-20) to all stats
+(5-10)% experience gained
Edit: Krischan, its not that. You are getting older :howdy:
If you are lucky, further messages of that kind will appear. If it's a hell game, it might be followed by "Diablo walks the earth", upon which the Diablo clone will replace the next super-unique monster (those which always have the same name) to be generated by the game which depends on where you or other players will go. If you manage to kill him, he will drop an Annihilus charm.
Good luck !
Edit: Hmm, others have been quicker than me, probably because I'm watching Star Wars on TV in parallel :lol: