Multimedia support in JDS is somewhat limited by what Sun is legally allowed to ship-- so, for example, it doesn't support the playing of encrypted (i.e. most commercial) DVDs, or the native win32 video codecs. You can find the relevant bits to do so on the net, but their downloading and/or usage may well be illegal in your country, and is therefore neither recommended nor supported in any way by Sun Microsystems.
JDS comes with Java Media Player, Real Player and Totem, which between them will play most common audio formats and several video formats (although I'm afraid I don't have a list to hand). Other media players such as XMMS, which support some additional formats, are available as RPMs on the JDS Community Website,
http://jdshelp.org.
No release date has been announced for JDS3 as yet, so I'd get into trouble if I posted one here
The Solaris version is in beta as you say (not sure if the Linux version is or not, yet), so the bulk of the development work is done... stay tuned