Taken from Personalizing Ubuntu by Keir Thomas:
The developers who designed Ubuntu’s desktop decided to keep the desktop clean of icons. This included relegating the Wastebasket icon to its own applet at the bottom-right side of the screen. Many people find using the applet a little difficult and miss the desktop trash can icon, which has been present on Windows and Mac OS desktops for more than 20 years.
The good news is that it’s easy to get the trash can back. Click Applications→System Tools→Configuration Editor. In the program window that appears, click the down arrows next to Apps, then Nautilus, and then Desktop. On the right side of the program window, put a check in the trash_icon_visible entry.
Alternatively, in the Configuration Editor, click Edit→Find and enter trash_icon_visible as a search term. Make sure that the Search Also In Key Names box has a check in it. Then click Find. The results will be listed at the bottom of the program window. Click the
/apps/nautilus/desktop/trash_icon_visibleentry. Then make sure there’s a check in the trash_icon_visible box.
Be careful when using the Configuration Editor program. It lets you configure just about every aspect of the GNOME desktop and doesn’t warn you when you’re about to do something devastating, so the potential for accidental damage is high!