I agree with Joe,

While there are some of us who can write both words and music.... it's not everybody's strength.
I've had some excellent lyricists who can sometimes sing me the tune they used to write their lyric...sometimes not. but can in no way play it on an instrument. and I'm not all that sure that they know much in the way of music theory.
But their lyrics flow with a rhythm that is strong. As I've said in the past....some of them practically write the music for me.
What is key for me more.... is that the lyricist understands how words flow. the natural rhythm and sycopation of how they are said.... that's where the difference between poetry and lyrics lies. Some words or phrases roll off the tongue easily...and some just don't.

frankly...to suggest that every lyricist can become a composer is a bit insulting to some of the great composers that inhabit these boards. If it's all just for fun...then great...go for it...
but if you're looking to compete in the commercial market...you can't be satisfied with just A TUNE...... you need a GREAT TUNE...
I'm not saying that some "strictly lyricists" here can't learn to play an instrument and go on to write some great music.... but to suggest that every one of them can, and should, (I know of one who did) I just don't think it's a requirement.

Harriet
http://www.soundclick.com/bands/pagemusic.cfm?bandID=24419

http://cdbaby.com/cd/ames

http://www.youtube.com/watch?v=SkGHmJQ-ODU