There is a thread
Creatine Thread over at bodybuilding.com (yeah, I know, don't trust everything you read there) that discusses this issue. Some quotes:
"Heating any amine (-NH2 group) containing molecule generally denatures it. Since creatine's structure is: (NH2)CN(CH3)CH2CO2.H2O, the same potential for denaturing exists"
"Creatine denatures at high tempertaures. Its ok to mix it in something that is hot enough to drink without burns"
"Creatine will begin to denature at about 45 - 50 degrees C"
On the other hand:
"As a result of this poor solubility, when the creatine gets to your gastrointestinal tract, the body tries to solubilize it...[or it] will just sit around in the pit of your stomach in powder form and eventually pass right out of you. So what the body does to remedy this is to suck fluids out of the cells of the digestive organs in order to provide enough fluid to dissolve the creatine. But...all this fluid that's sucked into the GI tract needs to quickly be eliminated and this leads to diarrhea. So in solubilizing your creatine, the GI causes some nasty bathroom situations...[and] a lot of the creatine is lost during such porcelain episodes.
"So what are some solutions? The first is to dissolve your creatine in a warm beverage. By doing so... the creatine is solubilized. And when consumed, it can be absorbed much more effectively without all the GI distress... Warm coffee, tea, or even just warm water will do just fine."
"Remember that your body temperature is 37 degrees C. Once you throw creatine into your stomach with fluid at room temperature it will heat up relativly fast as it passes through the intestine. Thus enhancing the solubility."
Remember, this is info from the web, so draw your own conclusions.
It would appear, however, from this and what Dan pointed out that dissolving in a warm (not hot) fluid is ok and may even be beneficial. Cooking may denature it, but studies typically show meat-eaters to have higher levels than vegetarians, so at least some of the creatine is surviving cooking.