The Intel processors used in Macs are designed to automatically shut down to prevent damage if they truly overheat (around 100C/212F - 105C/221F, depending on your processor).
iStat Pro will give you accurate readings of your temps, among other things.
Unless there is a rare defect in your Mac, your temps are well within the normal operating range, considering the workload you're putting on it. Websites with Flash content, games and other multimedia apps will put higher demand on the CPU/GPU, generating more heat. This is normal. If you're constantly putting high demands on your system, such as gaming or other multimedia tasks, expect temps to rise and fans to spin up accordingly. It's just your Mac doing its job to maintain temps within the normal range.
Your fans are always on when your Mac is on, spinning at a minimum of 2000 rpm (for MBPs) or 1800 rpm (for MBAs, MBs and minis). They will spin faster as needed to keep temps at a safe level. If they're spinning up without increased heat, try
resetting the SMC. Also, make sure you don't block the vents, which are located at the rear, near the hinge.
Learn about the fans in your Mac
Apple Portables: Operating temperature
