I am writing some code that constructs some rather large data
   structures and am having malloc fail to allocate space.  A little test
   program that just allocates space in progressively larger blocks seems
   to fail after about 8M have been allocated, regardless of the way that
   space is apportioned into blocks.

8MB sounds strange, but under 0.9 (and 1.0) the default data size
limit was (is) 16MB.  Try increasing your limit (using the `limit'
command if you use csh).