There is absolutely no question that distributing copyrighted materials without the authorization of the owner is illegal. The law is VERY clear on this point and anyone doing it is in violation. But regarding Napster, Perfectimage is right.
The question outstanding is whether Napster playing "middleman" is a violation of existing statute. One side says they facilitate the exchange of illegal goods, so are the same as a "fence." The other says their service doesn't cross that line, so is legal.
The courts haven't said what they think, though the only reason Napster is still active while the case is being decided is they plead hardship in response to the RIAA's motion for a cease and desist order. Very common legal practices, BTW, which in and of themselves prove nothing.
All in all this is a very emotionally charged subject. Sadly, that elicits arguments that have no legal basis. It matters not what terrible people the RIAA members have been in the past nor how much much they've ripped us all off. Look at the oil companies history. It matters not whether they've stifled artistic growth or alternative music. Any number of groups could be acussed of that. It doesn't matter how many people are doing it. Need I mention marijuana or speeding on the highways? No, none of that has any bearing on the subject except from an emotional perspective. They carry zero legal weight.
This is a politically charged issue as well. The RIAA wants the practice to stop. Ultimately it is about money. The RIAA wants to retain control of their property (it *is* theirs, btw) and the associated wealth and power it brings. There's an issue with the sheer numbers involved, literally millions of people. RIAA could start by making a few abusers acapegoats, but they know where that could lead. Forcing the closure of little Billy's MP3 site would create a horrific public relations problem. They'd be construed as bullies. Napster, OTOH, is a company, so it's OK to take them to court.
So, the issue at hand isn't whether violating copyright law is illegal; it is without question. Read the cover of any CD and then look up the laws they reference. There's no question about it. The issue being decided in court is whether Napster has done anything illegal. My guess is they will be found guilty on some level which is why they've already decided to crawl into bed with the devil by signing an agreement with BMG.
Ultimately the RIAA will win. They'll control all the Napsters one way or the other. They'll watermark everything is sight. They'll do whatever is necessary to ensure their survival by controlling their property. There's simply too much money and power at risk for them to not do anything and everything within their power to maintain control. Sadly, as is too often the case, the losers in the end will be the honest users. They'll get to pay for everyone elses' sins. Same as it ever was, same as it ever was...