System Information
Windows 7, 64 bit, GeForce 560Ti
Blender Version
2.63, 2.68, 2.69
Short description of error
If you make a linked copy of a metaball, it will be visible in the viewport in solid / wireframe mode, but not Cycles rendered preview. It will also not render in Cycles, but seams to work in BI
Exact steps for others to reproduce the error
Add -> Metaball -> Ball
ALT + D, move it a little
switch to rendered view or press F12
Pressing U -> Object and Data makes them appear again.