reuse of text between publications (CS) This probably depends on field, but in Computer Science  it is common for people to publish similar material in workshops, then conferences, then journals.  Each time it is acceptable to reuse some material from one level to the next as long as there is some new material (journals sometimes will publish verbatim but that's anohter matter).  

So, is it acceptable to borrow significant text from one conference paper to use in a second conference paper?   In my experience the answer is no, but I'm curious if there are any guidelines, and if so, whether they vary across disciplines.  

BTW, after posting I came across this: Attitudes towards self-plagiarism ... certainly related to this.  I commented there, and should have mentioned here, that a complicating factor with regard to reuse of text arises from the use of blind submission: if you can't say who you are, you can't admit to copying your own text.
I would say it depends:  


Some papers go from Abstract to Extended Abstract to Full Paper. In this case I see nothing makes it unacceptable. The same goes if you are extending your own results. 
I often see the same material copied in the Background (and
sometimes in the literature review) section with the same authors for
different papers. That is in addition to Definitions and common
notations as @Suresh said.
I believe in this case it is ethical and valid since Background (or Definitions) is not the paper contribution.  This ,in turn, has a disadvantage (and I really suffered from this): if you were not able to understand some parts of the background in the first paper, you will stuck there. In other words, you will not find another wording of it.


Returning to the question:  


  is it acceptable to borrow significant text from one conference paper
  to use in a second conference paper?


If the significant text is not significant part of the contribution then IMHO Yes. The word significant here is the key and it is ambiguous but let's say its 40% of the paper.  

The bottom line is:  
Removing the borrowing text, does your paper have a contribution by its own?  


I have seen the opposite where some papers are very similar to others and I think this is a bad habit. At the end, you are trying to discover new things not to copy them.