When upgrading from 11.10 to 12.04 and from Electric to Fuerte OpenCv are causing problems. You have to make sure you only have the correct version of OpenCV. Use Synaptic, apt, aptitude or something similar to check which packages are installed on you system. Once that is done do the following.

Make sure you have the file

/usr/lib/pkgconfig/opencv-2.3.1.pc

and that it looks like this

# Package Information for pkg-config
 
prefix=/usr
exec_prefix=${prefix}
libdir=${exec_prefix}/lib
includedir_old=${prefix}/include/opencv-2.3.1/opencv
includedir_new=${prefix}/include/opencv2
includedir_ros=/opt/ros/fuerte/include
 
Name: OpenCV
Description: Open Source Computer Vision Library
Version: 2.3.1
#Libs:  -l${exec_prefix}/lib/libopencv_contrib.so.2.3.1 -l${exec_prefix}/lib/libopencv_legacy.so.2.3.1 -l${exec_prefix}/lib/libopencv_objdetect.so.2.3.1 -l${exec_prefix}/lib/libopencv_calib3d.so.2.3.1 -l${exec_prefix}/lib/libopencv_features2d.so.2.3.1 -l${exec_prefix}/lib/libopencv_video.so.2.3.1 -l${exec_prefix}/lib/libopencv_highgui.so.2.3.1 -l${exec_prefix}/lib/libopencv_ml.so.2.3.1 -l${exec_prefix}/lib/libopencv_imgproc.so.2.3.1 -l${exec_prefix}/lib/libopencv_flann.so.2.3.1 -l${exec_prefix}/lib/libopencv_core.so.2.3.1
Libs:  -lopencv_contrib -lopencv_legacy -lopencv_objdetect -lopencv_calib3d \
       -lopencv_features2d -lopencv_video -lopencv_highgui -lopencv_ml \
       -lopencv_imgproc -lopencv_flann -lopencv_core
Cflags: -I${includedir_old} -I${includedir_new} -I${includedir_ros}