Not only does Shaidar Haran mean Hand of the Dark, when Moggy is summoned to Shayol Ghul, and Shaidar Haran puts her in the mindtrap, right before the myrdraal does this, the Dark One says, "MY HAND MOVES." Cause and effect. His hand moves, and puts her in the mindtrap. So, not only does Shaidar Haran's name mean Hand of the Dark, but the Dark One refers to him as his hand as well.

Also, I think that Shaidar Haran might something similar to an avatar, but his mind is independant of the Dark One's whatever he has (Doesn't he think that the connection to Shayol Ghul had to be severed somehow). Of course this could also be interpretted as the Dark One's frustration at being bound to the mountain (SG), but it doesn't really matter where the Bore is, because the actual spot in the Pattern where the Bore is, is no closer than any other place in the pattern to the Dark One's prison (the redstone doorway isn't any closer to Finnland than the Crystal Throne in Seanchan, but it is the gateway to Finnland), so being bound to Shayol Ghul is more like a physical thing for Shaidar Haran (he gets weak from being away). So he isn't the Dark One himself, but maybe he is a part of him (his hand?).

There is a possiblity on how the Dark One could have manipulated the fazed out quality of a myrdraal to make him an avatar (see my fazed out theory under True Power). By the way, someone post something there. There has been a disapointing 4 or 5 postings in that section, all but one mine, and the only other one by Mosleth who just says that Ishamael is Moridin again. Come on. This is an interesting topic! Anyways, the myrdraal is partly in the Pattern, and partly elsewhere. This is manifested in all the weird junk like fading (in shadows), making blurs in mirrors, and the cloak thing (not to mention others). The myrdraal are probably partly in the Dark One's dimension (prison), because they're evil and stuff. Well, the Dark One could link himself to the myrdraal with the black cords, and grant him the True Power. The more he uses it, and fades into the Dark One's dimesion, the Dark One could be fazed into the Pattern. So far the name Shaidar Haran is used, because only that much has been shifted (a hand), but by book 10, maybe it'll be "Hands and Feet of the Dark". (I don't actually think that Shaidar Haran is an avatar, but this just fleshes out other's theories).

It also appears that Padan Fain may be some sort of avatar. He can see Darkfriends at a glance (the Dragon Fang on their head, or something), has his little tricks (probably the True Power, and has been becoming increasingly more powerful as more of the seals break.

One last thought. In arguments for the seals being the link for the Dark One to taint saidin, people often use the argument that Lews Therin wants to break the seals. Listen more to the wisened Tamyrlin. It is a possibility that the Dark One causes the voices in channelers heads, and wants Rand to break him outta the slammer. I have no real opinion, just bringing up options.