I find it useful to distinguish between race tires (200 grams or less for folding clinchers) and racing/training tires (about 230 grams). You can use race tires on the back, but they'll only last about 1500 km (900 miles). But on the front, they'll go 3000 km or more. A good 230 gram tire (Vredestein Fortezza TriComp, for example) will last about 3000-4000 km on the rear (this assumes a 175-lb rider). For equivalent wired-on tires, add about 40 grams for the wires.
I personally don't think it's worth it to put a 200-gm tire on the rear; you really don't feel any difference. I've used one on the back for competition, but still didn't feel much difference. If you want to use really light high-performance tires, get a pair of tubular wheels and glue on some light sew-ups. I always rode tubulars in criteriums because they accelerated faster and cornered better than clinchers. However, I think clinchers work better in non-technical time trials.
L.