if i have an image that is 3000x2000 pixels and i export it to a jpeg file, it's still pixels. i did this and as a test export to 50 ppi as well as 500 dpi. Both jpgs look the same and are the same size when opened in preview. I understand the concept of ppi when it comes to printing. I even understand it in photoshop-when you change the ppi, the actual size of the image in the print preview changes. But in Lightroom3, changing the ppi in the export dialog does nothing. in fact, changing the ppi in the print module does not change the print preview either.