So, I went and talked with my metamour this morning...
That link to "am I in poly hell?" that Gala Girl posted pretty much summed up what she said to me. She said that she felt that all of his attention has been on me since our relationship started and that she's lucky to get any of his time or affection at all. She said that she's feeling increasingly edged out as time goes on and that when she's tried to talk to him about it, he's either not understood or doesn't care (my honest opinion is that he doesn't understand). I got the impression that she's slowly giving up on him.
But she also said that she understands this is a problem between her and him and that it really doesn't have anything to do with me. She said that she likes me and would like to be my friend, but that when I'm around, it reminds her how she feels like she's second place in his heart now and that is why she acts strangely around me sometimes.
I told her that I figured this was the case and that I hadn't taken any of her awkwardness personally (despite the fact that I was terrified she'd try to pull a veto... I did NOT tell her that part, by the way). That I was sad for the situation but didn't think I could do anything other than trying to keep her in mind when I ask him for things (ie, time) and ask him if he's taking care of her before he places all his attention on me. I also offered to babysit their daughter if they wanted to go out on a date.
I think that a veto is not likely now that we've sat down and talked about what is going on with her - I'm less afraid of what might happen with me and him and more sad for her and their relationship problems. It seems to me that there is a fundamental communication disconnect and some feelings of futility there. Either way, she's assured me that I've done nothing wrong and I'm less fearful than I was going into it.
In the mean time, he's decided to go to therapy. I hope that it helps. I do not want to see their marriage break up over this... but I also understand that whatever happens, it's not my fault - nor is it my responsibility - to fix things for them. I guess time will tell.