>	What could be the issues in using a mmap allocator than using the sbrk 
>allocator? 
>	Is it a nice idea to use mmap allocator everywhere?  

FreeBSD use mmap/sbrk for malloc(3). See

http://www.de.freebsd.org/de/doc/papers/malloc.html
or http://www.de.freebsd.org/de/doc/papers/malloc.{ps,ascii}.gz

Wolfram