I've seen a few articles with some vague mumblings about uploading songs to iCloud, but nothing definite. I think it's mostly speculation and guesses. As things stand now, nothing gets uploaded.
Songs are available only if they exist in the iTunes store; the song you've purchases, and if you purchased the Match service any songs in your library that can be matched with songs in the iTunes store. These songs are already in the iCloud. All you do is either get, or not get, access to them.
That becomes really obvious when you realize that any song you download from the iCloud service is the higher quality iTunes plus track, regardless of the quality of the track you have on your computer.
I don't think we are going to get a real answer to what can be done with actual uploads (for tracks that can not be matched) until iCloud is actually launched in full, or shortly before. But the rumors, yes, I think we'll have a few more of them.
Or I could be wrong. There's a lot of stuff being announced and leaked, and I may have missed a definitive answer somewhere. I can't read every article.