in Lightroom3, changing the ppi in the export dialog does nothing.
This statement isn't entierly true. If you turn on the checkmark next to Resize to Fit then the ppi setting at export *does* matter. Try two exports at the following settings:
Resize to width & height, width and height at 4x3 in inches, and ppi to 160
Resize to width & height, width and height at 4x3 in inches, and ppi to 300
You'll get very different file sizes as Lightroom will scale on export to get the right number of final pixels in each case (640x480 and 1200x900).