You will need to edit the "config.xml" manually to move SliderDock back on screen.
The file can be found here :
On XP:
"C:\Documents and Settings\(username)\Local Settings\Application Data\ SliderDock"
On vista/7:
"C:\users\(username)\AppData\Local\SliderDock"
Then open "config.xml" with notepad and look for ScreenPosition= (there should be 2 number after the =)
Change the 2 number to ScreenPosition="0,0".
Or if you do not wish to edit manually, you can just delete the config.xml but you will need to do all the customization in the dock settings again.
In the next version I will try to add an option in the taskbar icon to center the dock to the screen.