Hey all, thought I'd share a useful tip with anyone working on loopable N900 wallpapers.
One of the trickiest parts of creating a loopable wallpaper is getting the left edge and the right edge to match. To make it easier, photoshop has a filter called offset (found in filter -> other -> offset...) which shifts your entire image by a specified number of pixels. In an image 3200px wide, shifting it horizontally by +1600px (or -1600px) will move the edges to the center, making it easy to blend them together (of course, exactly how you blend is dependent on what kind of wallpaper you're making).