Oh god, is this a pet peeve of mine. I would link to a rant I did on it a while ago, but I really let the expletives fly, so it's not really appropriate. XD
If you want to sit in the store and read, like others have mentioned, there are chairs for that. Hell, there are plenty of choice floor spots where you won't be plopping your butt directly in all the other shoppers' way! It's the ones who sit there in the middle of a narrow aisle, don't even pay attention to those around them, don't even seem to care if you're trying to get to something around them, those are the ones I JUST WANT TO STEP ON.
And it makes me so mad, I can get pretty rude to them, and I honestly think I'm justified. If they don't move when they're completely blocking something I'd like to look at, I'm going to straight out tell them to. I think bookstores need to get in the habit of shuffling those kids along. Nothing wrong with reading in the store, in my opinion, as long as you don't treat the book like trash (I went into Borders right after Vassalord came out and the only copy was completely torn up. Stuff like that gets my little forehead vein twitching), but don't do so if you're being an obstacle to other people!
I tend to buy most of my manga online now. But I like to peruse the new stuff in the store, and I get coupons from Borders that I like to use. I pay attention to other people around me, I don't just stop dead in the aisle and not move when people try to walk by.
I think my main concern here is, these people need to realize that they are not the only ones in the world. MOVE for other people, man. Auuuugh. I really dislike those kind of people. And maybe they're not all so bad, so please, if you read this and think, "oh, I do that..." please just be mindful of other people around you and treat the merchandise with respect. Grab a few books you're interested in and go find a quiet, empty spot. Come back for more, but don't read directly on the floor of the aisle.