what's a fall girl?
and the title of the play "did you used to be dr. laing?" is it a joke or is that the grammatically correct way of asking what i would have worded as "did you use to be..."?


"fall girl" isn't common but it would be the feminine of "fall guy" which is. A fall guy is a kind of scapegoat, someone who to whom all the blame for some mistake or misbehavior (for which many people probably more important that the fall guy were actually responsible) is assigned by the others, in the hope of deflecting blame from themselves. A number of assistants to US Attorney General Alberto Gonzales, of increasing rank, have resigned in recent weeks. Apparently the hope with each of them was that he or she would take the fall and the scandal would rise no higher, e.g. to Gonzales himself or the White House. We shall see if the last one succeeds.
"Did you use to be" is correct but both "Did you used to be" and "Did you use to be" will look odd at first to English speakers. "Did you use to be" looks like it should be pronounced with a voiced s (like an English z) but in this phrase it gets the unvoiced s of "used to be" = "was". So the playwright had to choose between two alternatives, both unsatisfactory.

I can find no reference to "Fall Girl" so can only guess that it is intended to be the female equivalent of "Fall Guy" as stated above. I did some research into Laing but it doesn't provide any insight into "Fall Girl".
VinnyD -- I beg to differ with you on "use" or "used". It has to be "used". Would you say "I use to be famous?" or would you say "I used to be famous?". Not to belabour the point but "use" is present tense and "used" is past tense, which is what is meant here. Another more usual way of saying "Did you used to be" would be "Were you" but there is a slightly different connotation.
"use to be" looks very odd to me.
I did a site search of Associated Press sites and only found one instance:
"He's of no use to be an informant" - a quote by a non-native speaker and a different kind of "use" of course.
A search of "used to be" results in 35,000 pages.

#6 & 7:
Vinny was talking about negative statements and questions. In positive sentences, you may only use "used to", but when you use the auxiliary "did" then "use" becomes correct - as if it were a normal past tense verb. That
She used to work there.
She didn't use to work there. OR She didn't used to work there. (70,600 vs. 149,000 on google)
Did she use to work there? OR Did she used to work there? (7,010 vs. 649 on google)
Also, "Were you used ..." is passive voice, so that is a different subject.
<blockquote>Quote
<hr>It has to be "used". Would you say "I use to be famous?" or would you say "I used to be famous?". Not to belabour the point but "use" is present tense and "used" is past tense, which is what is meant here.<hr></blockquote> That's not the point, secretcover. There's a "did" in the sentence, which expresses the past tense. "Did you make tea?" is correct; "Did you made tea?" is not. Based on this argument, "use" would be correct and "used" would not. This only to say that the answer to mathilda's question is not as simple as you claim.