The indian rupee sign is pretty new (introduced this July if I read it correctly) and doesn't even have a Unicode position yet, much less a HTML entity.
Even when it gets a Unicode position, it will probably still take years until it can be reliably used on a web page, because the client computers' Fonts will need to be updated accordingly. (I could imagine a font-face
workaround with a custom font, though.)
Wikipedia uses an image file to display the symbol. It's far from good, but it may be the best workaround at the moment.
The generic rupee sign has three Unicode characters. See here.