This is an old trotter of a question, but I think it's more important these days than it's ever been.

The view I subscribe to, is that contemporary melodies are more adventurous than before. They tend to rely a lot on rhythm patterns with shorter note intervals (attempting to be more memorable, I think, as shorter intervals is easier to sing), due to the extended role of drums & percussion in contemporary music, and melodies doesn't start on the first beat as much as they used to.

So, there are MORE (but less dynamic) melody in contemporary music these days, and it is more rhythm driven than harmony driven. Melodies are also more fractured, broken up in smaller chuncks, I think. This is not my analysis, but one from songwriter Robin Frederick http://www.ustream.tv/recorded/5815303 (an hour well spent IMO). But I do agree with her.

If that premise is adopted before writing, I think this makes it more of a challenge to start with lyrics. Not impossible, and talents differ and all that, but to be able to craft contemporary sounding melodies, I personally feel it's easier to start with melody. Well, not just easier, but essential, in fact. If the song is more traditional, and doesn't have to have the contemporary sound, I think starting with lyrics is just as fine.

I often work on melody a'capella first, works out the sections, memorize it (I put chords on here), and then write the lyric to the (crafted) head melody.

Somehow I feel collaborating is more difficult due to this, as it takes a lyric writer that is very good with writing to melody. I used to write melody to lyrics, and it works fine, but I think the end result often ends up sounding dated, no matter how good it is.

I do think lyrics tend to be leaning more to rock in content due to this also, as stories are harder to write on top of a melody.. for me anyway.

So these days, I have lots to learn, but melody rules with me.