Yes, but you can use iTunes to restore all of your music back. You just have to build a Database in Rockbox, and the manual explains how to do this.
Your videos won't play under Rockbox, since Mpegplayer only plays MPEG-1 and MPEG-2 files... and rather poorly at this point, since all video decoding is done in software by Rockbox. We don't have support for the Broadcom video chip at this point. This
page explains more about the Mpegplayer plugin.
If there are other questions you may have, a search around the forums will point you in the right direction.