I just acquired the Rupee Medal in The Legend of Zelda: Skyward Sword. Supposedly this medal "makes Rupees mysteriously appear." But where do they appear!? Are they supposed to just politely drop themselves into my wallet? Do they appear when I defeat enemies? Or is it more frequent rupees when i cut grass/bushes? Or is the Goddess just trolling me!? I carried this worthless piece of junk around for a half hour and got bupkis!
My reading would suggest that the rupees appear during normal activities when you would already get rupees, such as cutting grass or killing enemies. You would just get more of them than normal.
I don't know of any hard numbers to support this, so it might not be an especially dramatic increase in rupees. It might become more noticable with more than one rupee medal, because anecdotal evidence would suggest that the effects of different medals stack. See this question: Do medal effects stack?