Camera hacker Bhautik Joshi, who brought us the brilliant DIY tilt-shift lens hack, has produced another great optical device. Detailed instructions on his site walk you through the creation of the Phone-O-Scope, an optical coupler that allows an iPhone to accept a standard SLR lens.
If I put my 1kg, Nikon f2.8, 14-24mm lens on a phone, it would need a shoulder bag to carry it...