Ran over to wowwiki to do a little poking around in the Worgen lore. Here is what I came back with, might be a long post. Hang with it.

-"They may not be from our world, but avenues exist between their home and ours and powerful magic can pull them here."
This is probably the root of all the extra-dimensional reasons on Worgen here, taken from the Book of Ur. Personally though, it seems like he could be saying either 'they don't exist here but we can summon them' or 'they may or may not be here at some point in time'. And really after all the fun the Bronze Dragonflight have run us through, Time-Magic is certainly not outside the realm of possibility.

-"The worgen's home is a dark place, a place of nightmare. If that world fosters locations safe from the cursed Worgen, then my research has revealed no such bastions."
Another line from the Book of Ur, and this really quite describes Azeroth back in the old days. The Silithid running everywhere trying to kill everything until the trolls (who were also killing a lot of others, even themselves after the Silithid War) split them up. But even after that you have the Burning Legion trying to enter the world. If I remember right Malfurion was one of the original Druids, so having a cult of Druidism pop up now wouldn't be too much of a stretch. There's also the Second Invasion by the Legion where they nearly took the World Tree until the Night Elves blew it up to stop them. Here is were the Worgen-Cult of druids could be fighting the legion, until Velinde and Arugal started warping them out. And don't forget, we have to go BACK to Hyjal in the time line because now the Legion would win. What changed in the past? Did some of the ancient defenders vanish, and where... when would they go? Recall you never see any Infinite Dragonflight in Hyjal as well.

-"...for although some Worgen possess powerful magic, they have made no attempts to reach Azeroth of their own accord. And for this, we are fortunate. "
Last one from the Book of Ur, promise. The Worgen don't need to get to Azeroth if they were already here! Also, they're job is to protect in the past, I doubt they'd abandon it willingly. Unless something in the future is more dire to Azeroth of course.

-"It is an ancient thing, a branch, twisted and gnarled of wood that could be older than even the Kaldorei. "
This time its from Valinde's Journal. Twisted ancient branch screams 'Druid Artifact!' to me, and if the Worgen-Cult of Druids formed about the time Druidism was being formed, that would make it a very old branch indeed.

-"The worgen fought savagely, as fit their primitive race, but their enemy was unflinching: the Lords of the Emerald Flame. "
Last quote! Once more from Valinde. Not going to go into the Lords of Emerald Flame, lets just assume for the purposes of this theory that the Lords are the Burning Legion. Now, the Lords of the Burning Legion were on Azeroth at one point. Hyjal. Archimonde was the head man here, but he also had plenty of Generals and what not here. The Worgen could have been fighting from the shadows of Hyjal until the time line sucked them out. And thus bring us back to deal with their absence. Also, Druids tend to get wonky if they stay in form and in the nightmare for too long, but there is also the tendency for the Demons to corrupt by mere presence. So slap a demon around long enough, your going to start getting side effects.

All of this, to me anyway, points to Worgen not being from another planet/dimension, but from another time. After fighting the Legion for so long (Not to mention the time magic, that can do wonky things to you. The Caverns of Time could erase your existence or have you walk back out yesterday. Its chaotic) hey could have had their minds degrade. And as for the shadow magic, its not Fel but, if you can't beat them steal from them. The Worgen-Druids could have learned shadow magic to help fight the legion, and that could lend to their now-degraded mind and corrupting presence as well. I have a whole lot of 'if this if that' here, but there certainly seems to be evidence towards to druid cult being the origins of the Worgen.