In my neighbourhood we often have the best viz under the ice. So it's not a question of IF, but rather a WHEN for most local diver's? A week ago I noticed that lakes allready have a thin layer of ice, so soon soon soon off I go...
Im my opinion there is no such thing as too cold water, but there can be wrong equipment. Obviously to dive with a wetsuite under the ice is a bit like extreme sport, but guys, You would not belive how many diver's do that quite often. (Not me offcourse... only DRY thank You
) Half an hour with 7 + 7 wetsuite in 4 C water is not a problem and just like Bent mentioned, it only "hurt's" at the beginning...
I remember one case where a friend of mine wanted to dive under the ice really badly. When we all got to the site it turned out that poor fellow had left his suite behind to the divecenter and was about to miss the entire dive. When someone made a joke, that he could have one wetsuite when the owner has come out from the dive, my friend accepted the offer happily... So he actually did stand next to the hole in ice, wayting for hi's really really wet and cold wetsuite and jumped out of his clothes and into the wetsuite in airtemperature -9 C just before the wetsuite had time to froze. Tied the rope through his BCD D-ring and slipped under the ice for an excellent 30 minute dive in 4 C water... And he's actually still alive and well. Maybe a bit loco tough.. 
So maybe sometimes it doesn't even take anything more but a strong will.
But seriously, icediving isn't anything supernatural and I suggest everyone should try it out atleast once. Obviously it can't be for everyone, but it is really something else, and You have no idea what a great part of diving You are missing out!! 
Cheers
Scubamoose