the problem with that is the fact that dry ice is -119F and will grow a protective layer of ice around itself making it slow to melt, which is not that big of a deal. while it is melting it is dissolving CO2 into your wart raising its acidity. this will aggravate the yeast when you do pitch it.
Now with that said i have used Dry Ice to chill my wart. what i do is get a small cooler and put a bag of regular ice and water in it with the dry ice. then put the hot brew pot in the cooler. next slowly circulate the wart being careful not to splash or ripple the surface. Combine that with a wart chiller and the temp drops like a stone.
Just use regular ice. Some people have done that with good results. I did it on my last brew and actually had too much ice (the wort reached 70F in about 20 minutes and still had a small iceberg in it).
I froze about 0.75 gallon of water in a plastic 1g milk carton, and then cut off the carton to get the ice out. I had about 3.5g of wort to cool, and also put the pot in an ice bath. The ice block was too much. Next time I'll try a 0.5g ice block, or split 0.75g into two separate containers. (multiple smaller chunks of ice will cool faster than one big chunk of ice).