I'll switch when it can convert image attachments to ascii inline, I don't want to open a separate program to look at all those adorable pictures of kittens in my inbox.
Download Apple's ASCIIMoviePlayer[1] and put the binary from the zip file in your path.
Add the following line to your .mailcap:
image/jpeg; ASCIIMoviePlayer %s && read WAIT
Enjoy kittens.
Of course, since it's ASCIIMoviePlayer it will also play videos if you add the appropriate lines to your .mailcap.
EDIT: Oh, and make sure your terminal is at least 180 characters wide. There doesn't seem to be a way to make ASCIIMoviePlayer output a smaller size.
[1]: http://developer.apple.com/library/mac/#samplecode/ASCIIMovi...