From the build output:

ar crs libecpg_compat.a informix.o
ranlib libecpg_compat.a
/bin/sh ../../../../config/install-sh -c -m 644 libecpg_compat.a '/Users/mike/Developer/Cellar/postgresql/9.0.0/lib/libecpg_compat.a'
Undefined symbols:
"_ECPGset_var", referenced from:
_ECPG_informix_set_var in informix.o
"_ECPGget_var", referenced from:
_ECPG_informix_get_var in informix.o
ld: symbol(s) not found
collect2: ld returned 1 exit status
make[4]: *** [libecpg_compat.3.2.dylib] Error 1
make[4]: *** Waiting for unfinished jobs....
cd '/Users/mike/Developer/Cellar/postgresql/9.0.0/lib' &&
ranlib libecpg_compat.a
make[3]: *** [install] Error 2
make[2]: *** [install] Error 2
make[1]: *** [install] Error 2
make: *** [install] Error 2
Exit status: 2

https://github.com/mxcl/homebrew/blob/master/Library/Formula/postgresql.rb#L51

==> Environment
HOMEBREW_VERSION: 0.7
HEAD: 1d1b637
HOMEBREW_PREFIX: /Users/mike/Developer
HOMEBREW_CELLAR: /Users/mike/Developer/Cellar
HOMEBREW_REPOSITORY: /Users/mike/Developer
HOMEBREW_LIBRARY_PATH: /Users/mike/Developer/Library/Homebrew
Hardware: dual-core 64-bit penryn
OS X: 10.6.4
Kernel Architecture: i386
Ruby: 1.8.7-174
/usr/bin/ruby => /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby
GCC-4.0: build 5494
GCC-4.2: build 5664
LLVM: build 2326
MacPorts or Fink? false
X11 installed? true

==> Build Flags
CC: /usr/bin/cc => /usr/bin/gcc-4.2
CXX: /usr/bin/c++ => /usr/bin/c++-4.2
LD: /usr/bin/cc => /usr/bin/gcc-4.2
CFLAGS: -O3 -march=core2 -msse4.1 -w -pipe -I/usr/include/libxml2 -I/Users/mike/Developer/Cellar/ossp-uuid/1.6.2/include
CXXFLAGS: -O3 -march=core2 -msse4.1 -w -pipe -I/usr/include/libxml2
CPPFLAGS: -I/Users/mike/Developer/Cellar/readline/6.1/include -isystem /Users/mike/Developer/include
LDFLAGS: -L/Users/mike/Developer/Cellar/readline/6.1/lib -L/Users/mike/Developer/lib -L/Users/mike/Developer/Cellar/ossp-uuid/1.6.2/lib
MAKEFLAGS: -j2
PKG_CONFIG_PATH: /Users/mike/Developer/Cellar/readline/6.1/lib/pkgconfig
Error: Failure while executing: make install
Please report this bug at https://github.com/mxcl/homebrew/issues

Also try:
brew doctor to check your setup for common problems.
brew missing to check installed packages for missing deps.