When I compile my app in Xcode 4.3.1 with iOS 5.1, I notice there is a strange behavior with background textures only on actual device. There is a 1px gap in between texture tiles shown in screenshot below.
My texture are 150×150 and 300×300 at 2x.
So far I’ve tested the same build on:
- Simulator iPhone/iPad both 5.0/5.1: No bug
- iPhone/iPad running 5.0.1: No bug
- iPhone/iPad running 5.1: Buggy

I’ve been getting the same problem since 5.1 aswell. I solved it by doing the following to the image in photoshop. You can probably do the same in another tool.
Load the file, select all, and copy to clipboard
Create a new file in photoshop that is the same dimension, RGB and 8bit depth with 72pixels/inch resolution with a white background.
Paste the image you copied in step 1 into this image
Save the file and use this one.
After doing this the file displayed correctly on the device and I didn’t have to resort to creating images the size of the display.