Personally I've been waiting for this for a while for this feature and it has finally arrived.
You can now play MP3 attachments straight from your inbox without having to download the file. As the screenshot shows a little "Play" link can be found next to MP3 files, when you click it you get a little pop-up with a flash MP3 player in it. Works like a charm.
I already used my Gmail to securely store some of my music, now I can also use it to easily play music on computers that are not my own without having to download files.