Great question.. most apps seem to leave folders with some data on the SD card; little to none seems to be left on the phones internal memory.
Looking at my SD card I see folders from several apps I have uninstalled. This isn't always a bad thing.. if you plan on reinstalling or upgrading an app; all your settings and info is still there without needing to do anything.
On the other hand; if you have no plans of ever reinstalling the app it is safe to delete the folder from your SD card. You can delete folders by either connecting to your computer via your USB cable or by using a file exploring app like Linda Manager.
— modified on Jul 23, 2010 3:28:04 AM