It's one thing when you buy the bike, piddle around for a few weeks, then come back and com plain; it's another when it's only a couple days. In fact, a lot of them will double their efforts to get you right when you go back for this reason; they want you to be a happy customer, AND they realize it's going to cost them if they DON'T.
Shops differ, but if you run into one that's more concerned about making/keeping the sale than satisfying you, the customer...look elsewhere. I don't work in a shop, but I deal with bikes all day long, and I generally only get irritated when it's something stupid -- like, you didn't listen to my advice the first time, now you want to bring the bike back because it's doing what I told you it would (YES, I get that! I've had bikes RETURNED for cracked reflectors!)
I had a guy (semi-regular, fairly new customer) show up at work this afternoon, complaining again about his new bike; he returned one on my day off a couple weeks ago, claiming it "fell apart" -- HE loosened the handlebar clamp to rotate the bar, and half-tightened it...that was ALL that was wrong. Today, he came in complaining about the shifting, how it was sloppy, shifted on its on and threw him off the bike (yeah, he's no candidate for Mensa). His rear derailleur cable had been loosened and was slipping, his front derailleur had slipped down the seat tube, and his crank had loosened JUST A TOUCH. He was back on the road in ten minutes.
Bike guys usually enjoy fixing the problems people have with their bikes; we all enjoy seeing someone having a good experience on two wheels!