I beg to differ. It doesn't do a frame any serious harm, but anything that's mild steel and not protected will rust, and seize if the bike's used regularly on salted roads. I've had to replace countless brakes, cables, miscellaneous nuts and bolts, chains - and the low dangly bits of rear deraileurs don't do too well if they have steel cages (and lots of low-end ones do). Some hubs are pretty poorly sealed, too (Shimano Capreo springs to mind) and a salty run or two will make them gravelly. Cheapo steel pedals' bearings are often barely sealed at all, and cheap ball bearings rust very fast indeed.
For this reason, I try to rinse off the bike after a salty ride home each day. There were lengthy periods of last winter, though, when that wasn't possible either because every source of water was frozen solid, or because if I did, the bike would be frozen solid, with immobile wheels, the following morning.
If you're going to commute through the winter, you have to accept that you'll be replacing components, because they've corroded, a bit more often than you otherwise would. Grease everything threaded before you screw it together, and grease seat posts too - or they'll never come apart again. Consider a single speed, hub gear or fixed wheel. Consider posh, stainless steel, sealed cables - for a bike used in salty weather it's a worthwhile investment.
All this runs into money, but when we're talking chains, brake cables (and lights), your life's on the line.
- no really.
