I just refreshed the [iphone]-tag-site, and I'm seeing all kinds of questions with what appears to be a grey overlay (see image). What is this for/why am I seeing this? I have never seen anything like this before..
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You've added a tag that the question uses to your ignore list. Any question which has ignored tags gets "faded" so you know it's probably not something you're interested in.
This is similar to how questions with tags you favorited get highlighted. If the quesiton contains both tags which are favorited and ignored, it will get highlighted yellow and faded.
See What do all the colors on Stack Overflow mean? for more information.