I wish I could play like Metheny in any state (singing or not).

Valentino,
 
I think you've nailed it.  My wife, who sings, always asks me about some aspect of a song, you know, with the words that go...
 
The blank stare.
 
I only rarely hear words while I'm playing-- I know the words only to songs that I learned while not playing.
 
Could it be a medical condition: anarthria musica?   Aphasia musica?
 
Brad

As I sing and play the bass, both my singing and my bass playing seem to be more towards autistic than artistic!

It used to be a problem for me in the way back when.  My two biggest bass influences, John Entwistle and Chris Squire, made it all seem possible that you could sing while playing a busy bass style.  I think you just have to detach the hands from the head.  Let the hands go and do their thing while the words flow out of your head.  Of course I've also had the same issue as j_gary and just up and forget the lyrics.  That's when you have to make 'em up on the spot.
 
It is always a pleasant surprise in listening to recording of yourself in a club and suddenly hearing a bass line you don't remember playing at all.

I am a great singer and a bass player !!!!!!!and,
 
 
 
I'm so happy nobody in this club ever get to hear me sing and play bass at the same time!!!!!
 
it would blow my illusion or delusion
 
David T (TLO)

David,
 
Amen to that!  (referencing my abilities, not yours)
 
John

I have played in a guitar / bass / drum trio for many years that only recently became a four piece. After getting sick of missing string parts, horn sections, organ parts and the like, I decided to teach myself to play a keyboard to try to enlarge the sound. Now through practice and gigging I play the bass with my right hand, (lefty) keyboard with my left, sing lead vocal and /or harmony and turn on and off a Digitech Vocalist harmoniser with my right foot all at the SAME TIME! So the next time you get frustrated just remember what I have to do to get through the night! I agree with the others, just practice and what seemed so insurmountable before will become so second nature you'll wonder what all the worry was about. Mike  
P.S. As I recall two of the most difficult lines to sing and play for me was the middle section of Roundabout and singing lead on Paul Simons  Late in the evening while trying to keep that bass line in the pocket.

I used to be in a 9 piece band that did Late in the Evening with your's truly on lead vocals.  A GREAT song, but I even had a tough time playing rhythm guitar and singing it at the same time!
 
Bill, tgo

Cool song, Bill!

I once read an interview with Geddy Lee of Rush, who can sing and play very difficult parts simultaneously. He said it just comes naturally to him, but for the really tricky stuff he needs considerable practice, and he makes sure not to think about it too much on stage.
 
And I can say yes or no while playing about 3 times out of 10 - I've improved!  ;-)