A few times I have found that persistent tongue sucking also goes along with
babies who have been sucking on one finger while in utero (mom's have seen
it on ultrasound and/or baby is still sucking on that one finger). 

Whatever the cause, it sure is frustrating.  I teach mom all the usual tricks
to get a baby to open its mouth, touching the lower lip, rolling baby gently
side to side, and moving babe away from the breast as soon as baby starts
sucking its tongue.  Also, cup feeding some of these babies gets them to start opening their mouths.  Placing the cup gently against the lower lip
generally makes them open their mouths, and the sipping brings out their 
tongues.  And at the very least, it gets nourishment into them while we're
waiting for them to figure it out.

Then, I also play games with the baby.  Two in particular:      1) holding baby
facing me, and moving him/her from lying down facing me, to sitting up, like
the dolls that say momma when you sit them up.  And as I sit them up I
make an open-mouth face, or say maa-maa.  (Moms think I'm utterly crazy).
2)  Again, holding baby facing me, I say "open wide" and do it myself -- when
babies start to imitate, I then start on "now stick your tongue out, A-a-ah," and assume the look I want to see.  As they start to imitate, I get mom's to start doing it, then bring baby to breast.  

Hope this helps.

Chanita, in San Francisco