With a heat wave sweeping the country it seems like a good time to review the basics of protecting you guitar from the elements.
Besides Pete Townshend (circa 1974) a heat wave is as likely to inflict damage upon the guitars of the world as any force on earth. However, it is not heat, per se, that is the real culprit. It is a rapid change in relative humidity that damages most guitars.
Your guitar can actually roll with changes from cold to hot fairly well if it the change happens slowly. However a quick temperature swing is almost always accompanied by a sharp change in humidity and this can require a visit to your Guitar Tech or can just plain ruin the guitar.
Now, avoiding these situations is generally not rocket science. Don't leave your guitar: locked in your car on a hot day, in your attic, in a greenhouse, etc... Keep it in a climate controlled environment around 68 degrees whenever possible.
Occasionally you may have need to play a gig (or worst case put your guitar on a tour bus) on a blazing summer day. Don't fret. This shouldn't be a problem. Unless it is crazy hot (lets say over 100 degrees) risk of delamination (worst case scenario) is minimal. All guitars and especially cheaper guitars are at risk for unpleasant problems such as neck twists or fret sprout from a summer day gig. This is because the wood in less expensive guitars is generally not dried down as well. The moisture content in the wood reacts more strongly to changing humidity.
The most important thing to remember if your dealing with any guitar that has been exposed to the elements is to let is get back to 68 degrees as slowly as possible. If it is in its case, do not pull it right out to see if it is OK. If you have left a guitar in a hot car and then bring it directly into an air conditioned house and open the case to inspect you may do the majority of damage to the guitar in that final instant. Resist the urge to whip open the case. Let it cool slowly. If you can wait 24 hours... do it.
The same holds true if your axe gets very cold in the winter. A guitar can easily handle temperatures in 30s or even 20s (F) but if it is brought right into a 70 degree room after getting that cold, you are likely to watch the lacquer crack right before your eyes.
So there you have it. Heat is bad for your guitar but quick changes in relative humidity can be lethal.