There are many words that people often mispronounce when they say them aloud, because they know them only from reading. Cache (pronounced "kash") and cachet (rhymes with "sashay") fall into this category. Radio reports sometimes mention "weapons caches" – and pronounce it "ka-SHAYS." Oops!
Conversely, I've seen in print comments to the effect that this or that business school, for instance, has "great cache" or even "caché." Cache and cachet seem to pop up in each other's place like twins continually mistaken for each other in a Shakespearean comedy.
Just to be clear: Cache is a hiding place for food, ammunition, or similar supplies, or the supplies themselves. It has often been spelled the way it's been pronounced. Cache is associated particularly with explorers of the American West – Lewis and Clark, for instance – and the Arctic.
A cachet, on the other hand, was originally a seal – a king's personal seal, as distinct from an official seal. Then the meaning stretched to cover any indication of approval conveying great prestige (e.g., "Istvan's new place has won the cachet of the Best of Boston award for the Best New Afghan-Hungarian deli").
From there cachet has come to refer to the prestige itself.
Cachet has its very dark side, though. A letter of cachet (lettre de cachet, in French) was a letter by the French king, under his private seal (going back to that original meaning of cachet) containing an order, often for someone's imprisonment or death.
A happier specialized use of cachet is a philatelic one – the little advertising message or other motto on a postmark or a postage meter impression, "Season's Greetings" or whatever.
If there seems to be a family resemblance between cache and cachet, are the two words related? Yes. Both derive from the French word cacher, to hide.