Beginning 4/15/2010 if you use Firefox or the Google Chrome browser you can drag and drop attachments into your email. Drag and drop attachments onto messages on the Gmail blog has some nice screenshots (minis below) of exactly how simple the new process is.
Its not working in FF 3.6.x for me, but I do have lots of add-ons. It worked great in Chrome, though. Great thing is that you don’t need to go into Gmail Labs or settings, its already activated.