Thanks for the quick response. For what I'm working on, I have to use PNG. When I save as PNG, the Alpha Channels checkbox goes unchecked and becomes disabled. Save for Web only has the PNG-24 option, which (I guess intentionally?) doesn't include the alpha channel. I don't see a way to save the png with the dropshadow and transparency.
I did just try to save at TGA per your advice, and it seems to work, except the dropshadow is white. I'm using a file created by following this tutorial to a T, so I'd inverted the alpha channel once I created it. I expected the dropshadow to be black. Any ideas?
Again, I thank you for your help.