In the US, you can legally rip audio CDs that you own for playback on devices like your PC and portable MP3 player. In some countries (mainly European), you can also legally share some of your music with others as long as there are no commercial undertones. The US is very intolerant in that regard.
I must admit that I really don't pay much attention to the nuances of US copyright law. I will rip audio CDs that I borrow from the public library with the intention of listening to the material after I have returned the physcial media. I don't lose much sleep over it because I am a very good customer of the industry, spending easily $1000 a year on music.
I have the very liberal attitude that I should be able to listen to any publically-released album at any time and on any device...without having to buy the physcial copies of the estimated hundreds of thousands of titles that have ever been released. I don't have the money (or room

) for that...nor does anybody, really. Not that I expect this ability to be free or even cheap, but time is finite and I get somewhat aggravated that I still have to pay about $15 for a new CD, even though I already have 600-700 (as that collection grows I have less finite time to devote to each). $15 is fair to the average person who may buy 10-15 CDs a year, but ouch, it hurts for somebody like me who buys 10-15 a month...that's why I have to buy most CDs used.
If a title is OOP, I still want to hear it. I shouldn't have to blow a wad on eBay to hear some rare album. Consider the first three albums by the Comsat Angels, a new-wave group...on eBay their CDs go for $75-100 a pop because there aren't enough copies to go around. It is frustrating because you only have two choices: pay a king's ransom for the physical media or download the MP3s.
So in my visionary outlook, someday you'll be able to legally download any album that every existed across your fat broadband pipe and be able to play that digital album anywhere without restrictions. Going to the music store to buy physical media will be antiquated. You could conceivably have 25000 download albums, legally. All this at the cost of about $99/month. Can't afford that? They'll be more restrictive plans for less. I am ready for this future, but I'll have to wait a good decade or so before it happens.
Actually, emusic.com has something like this already - at $15/month. Unlimited legal downloads in MP3 format (unsecured) for about 5000 artists, some fairly well-known. The problem is that they use 128kbps encoding, I think.