reuse of text between publications (CS)
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.