I think it would be really nice to see if an answerer has tag-related badges on those tags that have been applied to the question. For example, if I asked a question with the tags [php], [symfony-2.0] and [doctrine], and the person who answers my questions has a golden [php] badge and a bronze [symfony-2.0] badge they would be shown in the same place as the user's picture, reputation and badge count.
I think it really would be nice
Yes, so do I, but does it really help us? This reminds me of the "don't get sweet talked into building a truck" section of Jeff's blog post.
SO gauges answers by "post quality", not "poster quality". "Post quality", in turn, is gauged by votes. One should not use the user's rep/badges/etc as a way to gauge post quality. In fact, there have been feature-requests in this direction, asking to hide rep/badges since they don't convey anything useful.
Also: I'm a novice at jQuery (Secret: I learned jQuery after I started using it in SO userscripts*). Yet, due to the high volume of simple RTFM jQuery questions, I think it wouldn't be too hard to get a jQuery tag badge by FGITWing all jQuery posts. I bet this applies to other badges as well. So the tag badge isn't useful at all for judging arbitrary post quality.
*MSO, please don't kill me
I was just thinking of the same problem when I found this topic. I've seen several occasions where different answers were provided for the same problem, and the user ended up implementing a, let's be blunt, bad and ridiculous solution to his problem. I think there's certainly added value in showing relevant tag badges next to answers, as a 'general rough guide' to credibility of the answer.
The argument presented before that it's relatively easy to get tag badges in popular tags is irrelevant - because you've then still proven to be able to get a lot of upvotes in them, meaning you have some kind of credibility on the subject. Also, I've just gained my own first bronze tag badge at 5k rep. I highly doubt there's that many people that have 'easily gained' tag badges, especially silver or gold.
Another argument is against the space it would cost. I also think this is a non-issue, since there's honestly not that many people with tag badges in all relevant topics, and it could easily be added without taking up more space in a regular user's layout:
The tooltip could then show something like:
Jon Skeet has earned 52959 upvotes for 4302 answers in the c# tag.
And clicking it should then link to the user's answers in that specific tag, so the OP can further inspect the credibility of this user and his answers.