byte-compile / cl package / warnings

[David Reitter]

Hi,
I often get warnings like

"Function `set-difference' from cl package
    called at runtime"

when byte-compiling stuff.

Isn't it safe to call stuff from the cl package from byte-compiled code?
I looked into the byte-compile source, and I found that the cl  
package documentation says that it's safe to require 'cl.

So why do I get this warning, and do I need to react?

Thanks
Dave