Buffalo has introduced their RMUM Series USB flash drive in Japan, which in other words is probably the smallest in the world.
Infact it is not exactly a flash drive but a microSDHC card reader bundled with a 1, 2, 4 or 8GB microSDHC memory card. In other words it may be the smallest microSDHC card reader.
Size:15 x 22 x 8mm at just 2.3g.
Right now it will available in December, in Black or White Color Range.