Hatchery winter steelhead do on average spawn earlier than natives, but some overlap occurs. If summer steelhead are present, they may spawn at the same time and interbreed. Keep in mind, one of the biggest problems with hatchery steelhead and salmon is that the rivers have a limited capasity to harbor all the juvenille salmonids. If you throw 100,000 hatchery smolts into a section of river with say, 5,000 wild smolts, you've just decreased the wild smolts' ability to eat, hide and survive.
Another factor that has been well documented in rainbow trout is that fish stocks from different basins have differing genetic resistance to diseases. Oregon's Metolius River is a prime example. Before the stocking of hatchery trout was discontinued in the mid 90's, ODFW documented a 75% decrease in genetic resistance to a particular parasite found in the area, in wild rainbow trout. They were able to document this by comparing wild fish above and below an impassible falls which the hatchery trout could not get below. Since discontinuing the hatchery program, the wild trout population in the upper Metolius has strongly rebounded. Steelhead are genetically identical to rainbow trout, so the same genetic disease resistance problem is believed to occur with them.
I've got to say, I find it disturbing and hard to accept that so many people dispute that hatchery fish stocks harm wild fish stocks. To a large extent, it's obvious and logical. If you disturb the natural order of things, you're going to disturb the natural order of things. I can except that some people want more (hatchery fish) and other people want better (wild) fish. To deny that one type has an effect on the other is to deny basic laws of science (for every action, there is a reaction).
This brings up my final point : Why don't we leave it up to the fisheries biologists to decide fisheries issues. Contrary to what a lot of fishermen think, they really do know a lot about fish. Those fisheries biologists that I know personally are all over this issue (with real science, not Fisherman-Joe's personal opinion. Their opinions deserve our attention and respect.