I can't say I've heard of an iPad screen cracking from the heat.....but it just isn't good for them to be left anywhere hot. They will shut down when overheated even from using them in the sun on a hot day.
Have a look to see if you can see any point of impact from a drop or if it's been bent out of alignment if something heavy has been placed on them, like sitting on one.
We have had reports from members of a screen cracking for no good reason, I'm thinking as with anything, you can have imperfections in any type of glass, but Apple themselves have shown they don't tend to believe this.
I myself had a shower screen explode as I was walking past it, it was explained that any imperfection in glass along with change in temperature can trigger this event.
I hope this helps you with your decision on what to do.
Some more info.
The best solution we have for fixing a broken iPad is to make an appointment with the Genius Bar at your nearest Apple Store....you will not need your receipt as they can look up all relevant info at the store. They will most probably offer to replace your iPad but for a cost...it's normally half of the original cost of the iPad. If this is fine by you, make sure you do a complete backup of your iPad before you go.
There are other businesses that can do this for you which will probably be cheaper but be aware that this will void your warranty, but then again your warranty would already be voided because of the cracked screen. If you do replace with Apple, your warranty is reinstated. I have also heard many members who have gone the cheaper route then find that their WiFi will not work as this has been damaged by the replacing of the screen.
In addition, you can also contact Apple customer service. Here are some great web sites to aid you in your repair search:
Contact Info:
http://www.apple.com/contact/
Apple Support:
http://www.apple.com/support/
Check for Warranty:
https://selfsolve.apple.com/GetWarranty.do
For those of you not living in the USA, here is a link whereby you can select your country and then be directed to the Apple site for your country:
http://www.apple.com/choose-your-country/