The Question (SPOILER!!!) It is said that Harrys invisibility cloak is "impenetrable" yet a number of charactors can and have seen through it through the books (Dumbledoor and Mad Eye for example). Anyone got any suggestions on how they do this if the clock is impenitrable???

I took it that Dumbledore, and Mad-Eye picked up on Harry's presence some other way (noise, things shifting, etc.) kind of the same way Malfoy caught him on the train in Half-Blood Prince. Not 100% on this, it's just my take on it.

They meant that the invisibility could never wear off of the cloak since it was from one of the Peverell brothers. Plus, J.K. Rowling has said that Dumbledore used the spell "Honehem Revelio" (sp?) which reveals humans, so he could be able to see Harry. Dumbledore could do magic without speaking. As for Moody, it is not revealed how he is able to see through Harry's cloak. We know however, that it is the work of his magical eye which can see through walls, doors, and Invisibility cloaks.

I took that statement to mean that it is impenetrable to human eyes, however Dumbledoor was in possession of it (and the elder wand) for a long time, he could have studied it. And mad eye was not normal, he had a magical item created specifically for him.

I am sorry but I dont agree with any of the other answers. The Honehem one is possible but surely, a great wizard like Dumbledore wouldnt need it? Its all about magical energy. Dumbledore could sense Harry through the cloak because Harry was emitting magical energy. Because Dumbledore, as mentioned earlier, is an exceptionally great wizard, he could pick up the energy wafting around him and know which direction it was coming from. Harry is also a host for Voldemort's seventh part of the soul. So HUMUNGOUS energy would be emitted by it and it would easier for Dumbledore to detect Harry. The cloak is impenetrable. That means magical energy cant go inside but can come out. Harry casts spells from under the cloak without damaging it right? But if somebody casts a spell at him or it when he is under the cloak, for example "Accio cloak!", it doesnt work. Its like the boat in Harry Potter And The Half-Blood Prince. It could detect the magical energy. Now, about Moody. Remember his magical eye? Its an OBJECT right? Its an object with magical power in itself. No wizard is using a spell to make it work. Harry's cloak was made by, apparently, Death in such a way that no spell could detect it. But since Moody's eye does not work on a spell, but built in that way, it could see Harry under the cloak. This is like how they mention in Harry Potter And The Deathly Hallows that brroms and Thestrals cant be detected as they have magical power in themselves. A spell is not being used. Hope this answer was sufficient!

Why could the most powerful wizard possibly ever (THAT possessed and studied the cloak in question for a long period of time) and a man with a magical eye see through Harry's cloak? Hmm lets think about this for a minute. A magical eye... most powerful wizard... understanding of the cloak and what it really was... I really don't know. *hint* sarcasm.