The spinning "beach ball", as it's commonly referred to, can appear for a number of different reasons - heat is not one of them.
Actually that is incorrect. Nearly all Intel processors have thermal protection. This includes a temperature sensor built into the processor die as well as thermal throttling and thermal shutdown.
If a processor overheats (over its Tcase specification), it will lower its clock rate then halt.
Trust me, the Apple engineers are competent.
Engineers cannot anticipate every environment nor every possible failure mode. For example, MacBook Pro laptops placed on a soft, deformable surface like a couch or futon will overheat because the heatsink vents on the back near the screen hinge can become blocked. Ditto with dust, grime, pet hair, tobacco smoke, etc.
Manufacturing defects are a possibility. Consider the failures of adhesive thermal compound pads in numerous laptop brands. Or the lithium ion battery fires caused by metal contamination during assembly. Or the capacitor plague caused by bad electrolyte recipes. Bottom line, mistakes happen even in the best engineering teams and manufacturing facilities.
That said, the most likely causes of this symptom are:
Run away process. Use Activity Monitor or similar program to see if any program or service is using a large percentage of processor time or hard drive activity.
Insufficient memory. Use Activity Monitor to determine the amount of free (unused) memory. If it is consistently low, then a memory upgrade might be an inexpensive solution.
Insufficient hard drive space. Check the available space on your boot drive. If it's less than 20%, consider deleting unnecessary files, moving seldom used files to another drive, or upgrading to a higher capacity drive. Utilities like JDiskReport can help you locate large folders and files.
Hard drive failure. Read or write errors can manifest as long delays even though the storage device appears to work. Use a SMART utility to check its stats and a benchmark utility to check its speed. Listen carefully to the drive for a high pitched whine or grinding noise. On the filesystem level, try a filesystem check (fsck) using an OS install DVD if possible.
System overheating. Check the air vents on the computer to ensure they have no obstructions. With the computer turned off, use a can of compressed air to blow the vents clean. If you have long haired pets or smoke near your computer, additional cleaning may be necessary.
More things to try:
The Spinning Beach Ball of Death
The Spinning Beach Ball of Death
