Yeah, this is true. I don't really like the term "half vampire" for her either, but I suppose it's the closest you can get to something describing her. The impression I got from the books (and it's been a while since I read some of them, and I may have it wrong) is that the Red Court Vampires are related to something in the Nevernever. I thought of it as working that when you are bitten by a Red in an attempt to turn you, your body undergoes changes preparing to be a vampire and providing you with strength, a desire to drink blood, and whatever else. Then when you actually drink blood, the whatever/vampire-thing from the Nevernever is admitted access into your body/mind and you become a full Vampire.

Eh, wall of text FTL. It's probably not even right. >.>

As for what could have happened in the time she's been gone, yeah, a lot could have happened. I actually read through the entire bloody thread over on the official forums a few days ago, I'll just briefly sum up some of the possibilities they have come up with there.

Fellowship of St. Giles
They forced/ordered/coerced Susan into keeping quiet about the child, which is actually Harry's.

It is possible that it was them who took the child.

Susan's "our" could mean a child collectively belonging to the Fellowship somehow. Like a half-turned 5 year old. This, however, is highly doubtful as Jim Butcher has stated that Susan intended for Harry to take her statement as... well damn, semantics fail me. Susan wanted Harry to think that the kid was his flesh and blood. Or at least that's how everyone is taking JB's comment.

Memory Loss/Alteration
Headaches in Turn Coat are related to Harry trying to remember something relating to child instead of Lash coming back.

Susan could genuinely believe what she says on the phone, but that is a result of her memory being changed somehow and it is not Harry's child.

See Mab comment in "Other"

Other
Non-magical related possibility is that Susan kept finding a reason not to tell Harry. Boring, yes. Realistic? Maybe. Unlikely? Hell yeah.

Susan turned into a Full Vampire and is actively trying to lure Harry out in order to kill him or set him up for a trap for the Red Court. There are few things she could say that would get him more flustered and ready to rush off without thinking.

Or there could be a magic-related reason that Susan didn't tell Harry. Maybe kids like that are automatically put down by the White Council for all we know, and she didn't want to risk it.

Mab is somehow influencing the situation in order to force Harry into a position where he will accept the mantle for Winter Knight. Could also involve memory loss/alteration since she's done it before. One possibility is Mab providing protection for this child (who is sure to be a target for various reasons) provided Susan doesn't tell Harry about it.

Susan is doing some sort of role playing thing with Butters and the Alphas. This apparently happened in one of the short stories I haven't read.

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Might be a few more I forgot, but there you go. Some of them I wouldn't have thought of.

EDITED TO ADD my personal theory:

Maybe Cowl or Mab or *insert other bad guy* wants to attain immortal life and will come after the child because some obscure connection to Maggie/Harry/Vampire!Mother could prevent this, and Susan will stand in front of the child and refuse to move when said bad guy comes to kill him/her, at which point she will be killed, and this sacrifice of her life for her child will provide the child with the protection of a mother's love, which will be instrumental in allowing the child to grow up and put an end to the Black Council. Just saying. Also Susan didn't tell Harry about all of this because it was prophesied. Or something.

And yes, I'm joking.

 

What's that a quote from Big D? Sounds pretty fuckin' awesome.

And yeah, Cruentus, I tend to agree with you about why she wouldn't be lying to him. The only exception might be if she was a Red and they had some sort of ... I don't know, magic thing going on that required Harry to be as emotionally screwed up as possible, but that's pretty damn unlikely.

 

He makes the Merlin and Rashid nervous too. We have the advantage of being in Harry's head, so we know he's a good guy, but they don't. Like folks have said -- he's a wildcard, he's starting to get powerfully magical, he's accumulating interesting powers like Soulfire, he's in possession of two of the swords, and he's forging some unexpected alliances (Demonreach, the little fae banding together, etc.).

Maybe the question should be, who ISN'T made nervous by Dresden?