[mr] Has posted an interesting article about Apple patents geared towards attachable lenses to its iPhone camera phones.
Currently the most popular and best brand of lens attachments I would say is the Olloclip 4-in-1 Lens Solution for iPhone. They make a whole range of lenses from Fisheye to Wide, Macro, Zoom and a Polarizer. Those are at least the ones that I have. They are fun but you have to be careful they don't fall off if you don't own the special case.
According to Macrumors, the attachment on the phone will be a bayonet style which will allow different lenses to pop on and will be a lot more stable without the need for an external case or adapter like with Olloclip. The patent currently did not say whether or not they would be making lenses as well. Hopefully they do not, so great companies like Olloclip can stay in business since they really do have a nice product.
But come on, let's get serious. This will never happen for the iPhone, Apple would never compromise the clean design of their iPhone with some bayonet attachment clip around the camera. If anything Apple will explore some sort of weird iPod camera type thing that is completely separate from the iPhone or even iPod product lines.