Daily Water Temperature Readings
All - this website ( http://waterdata.usgs.gov/nwis/uv/?site_no=12417610 
graphs the water temp in the lake every day in Celsius ... currently still between 53-54 degrees F (scroll down to the 6th graph). Apparently the guage is about 9 feet below the surface so the real feel should be warmer than what this reads. I'm hoping the warmer weather coming next week (per accuweather.com) will help get the lake over 60 before the race. And now, I can check the temp daily to stress over it. 
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If it's sunny for just a few days before the race, that can raise the swimming temperature (the top 12-18") up to 5 deg F in a matter of 48-72 hours.
Of course, it's better to stress right now about making sure you bring racing clothes which will be good for the range of temps possible: 98 - 48 F, the temps from the first race, and the last one! As WTC says, "Anything is Possible!"
* have the right gear.
* mentally be ready to suffer...think ice cream headache, but no ice cream.
* know it will affect your ability to digest / desire to eat the longer you are in there...folks going longer than 90 minutes need to be wary.
P
Like Al said, the water temp, as well as the temperature and weather, can change dramatically in just a few days at CDA. In '03, on Wednesday, I had one of my roughest swims ever. Very, very windy and choppy, temp was probably low 80's. Race day was calm and high 90's.
'08 the water temp was about 54 on Wed or Thurs. It had "warmed" to about 58 by the race. So have the right gear and be ready, but really not worth stressing about or thinking too much about until 0645 on race day. Nothing you can do about it, it is what it is.
General statement regarding weather conditions at IM's: most of us had ridden in a wide variety of weather: hot, cold, calm, wind, sun, rain, etc. You just deal with it, sometimes just suck it up. But, for some reason, if it's an Ironman race day that isn't absolutely perfect, people wig out and put on all kinds of whack layers, jackets, rain gear, etc, only to be stripped off at mile 20. More importantly, it gets in their head and either stresses them out before or during the race.