Back in October Lev Grossman did a post about writers who read fiction while they are writing their own, vs. those who read no fiction. He calls these two groups Thieves and Soloists. He himself claims to be a thief, and I'd put myself in that camp as well.
Grossman seems mostly to "steal" stylistic or otherwise intangible elements of the fiction he reads (such as the "pure wonder" he finds in C.S. Lewis). That is also my experience. To put it another way, I use previously published fiction as a kind of permission slip to do things I wasn't sure it was OK to do. Oh, you can go on at great length about the childhood of a minor character. You can switch point of view in the middle (provided you do it with clear intention) and then switch back again. You can include drawings and diagrams. Etc. I think this is related to what Grossman says about reading Neal Stephenson to "raise my own game." You're shooting yourself in the foot (I believe) if you avoid authors you believe are truly great, because you fear they'll intimidate you into not writing at all. I say, read them, and raise your game.
Sometimes my thievery is even less direct. Kind of like playing the radio in the car, the "sound" of another person's prose in my head seems to skim off my excess intentionality--which is maybe the same thing as the superego/editor voice we're all supposed to stifle until much later in the process. Anyway, it's that urge to try too hard, to labor over the surface of the prose, to show off, even. The other author's prose drowns that out--so that, as I'm reading, suddenly I'm reaching for my pen and notebook to jot down an idea that's completely unrelated to what I'm reading. I'm not stealing an actual plot point, or character trait, or even a metaphor. Rather, some notion--some concept or sound or emotional experience--has been freed, by engaging my overly conscious mind with something sufficiently challenging. I think it goes beyond distraction, but that's a thought for another time.
All this is why I don't worry to much about what exactly I'm reading at any given time. Just because I'm writing a (sort of) mystery doesn't mean I feel I should be reading Raymond Chandler. Although maybe I should. I figure whatever I'm reading at the time is something I need to be reading, for reasons I may not consciously understand.
I think in the past I made more of an effort to avoid reading fiction while writing (and because I was writing most of the time, I read little fiction). I suspect this was a sort of juvenile fear about not being "original." But I actually think the Soloists steal too; they just have better memories for what they've read in the past.