Tac,
The way it has always worked here is:
1. Lyricist post lyrics and own the copyright.
2. Musician/Songwriters see something they like.
3. Musician/Songwriter contact lyricist and ask for permission to use the lyrics. They may create a working bed track idea to pitch first.
4. Musician/Songwriter and lyricist agree to collaborate. At that point the finished song with both parts belong to both parties. Both listed as copyright holders.
5. If someone were to "lift" a song and not give credit on this site they would be gone!
Just as a note. There are a lot of witnesses that watch these songs grow. I am sure that would help in litigation.
There is also a Creative Commons license that can be applied to a release. You can read about that here:
http://creativecommons.org/Thanks,
Nick