libunwind is available as a package (pkg_add -r libunwind) on FreeBSD 8.1 amd64.
It is not available that way on FreeBSD 8.1 i386.
When I download from http://www.nongnu.org/libunwind/ I cannot build it.
The error is In file included from ptrace/_UPT_elf.c:4: ../include/libunwind_i.h:65:5: error: #error Host has unknown byte-order. * Error code 1
This appears to be a ./configure error. Is there a way to force this information at configuration time?
According to this page: http://www.freshports.org/devel/libunwind/ libunwind is available for i386 and has been fixed on August the 12th.
If the package is not available, you can then probably build it from ports.