New Scientist Tech - Breaking News - Physicists draw up plans for real 'cloaking device'
It's not quite the cloaking technology in Star Trek. This makes use of optical properties of special materials, wheras the Klingons and their friends tightly packed and modulated energy fields to achieve a similar result.
I think the closest thing to this in Trek is the genetically enhanced agents of the Suliban Cabal. Their colour-changing skin and biomimetic garments are not sophisticated enough to produce the kinds of effects we see in the show... but some crazy optical properties would do the trick.