Is your image_science throwing an error when you try to install Rails 3 on a new machine for testing and updating purposes?
My error was something like:
gcc -shared -I/usr/local/include -fPIC -O3 -ggdb -Wall -Wextra -Wno-unused-parameter -Wno-parentheses -Wno-long-long -Wno-missing-field-initializers -Wpointer-arith -Wwrite-strings -Wdeclaration-after-statement -Wimplicit-function-declaration -fPIC -L. -rdynamic -Wl,-export-dynamic -I /home/USERNAME/.rvm/rubies/ruby-1.9.3-p547/include/ruby-1.9.1 -I /home/USERNAME/.rvm/rubies/ruby-1.9.3-p547/include/ruby-1.9.1/x86_64-linux -I /home/USERNAME/.rvm/rubies/ruby-1.9.3-p547/include -L/home/USERNAME/.rvm/rubies/ruby-1.9.3-p547/lib -o \"/home/USERNAME/.ruby_inline/ruby-1.9.1/Inline_ImageScience_3307dc78f4693be553254d3891179fbb.so\" \"/home/USERNAME/.ruby_inline/ruby-1.9.1/Inline_ImageScience_3307dc78f4693be553254d3891179fbb.c\" -L/usr/local/lib -lfreeimage -lfreeimage -lstdc++
I searched up and down for gcc no such file or directory errors after trying to run the command on my own. Then I realized that Rails and passenger might be eating some of the actual error. After quickly loading irb and running "require 'image_science'" I found out that I did not yet have FreeImage installed.
On Ubuntu this is a simple matter of:
sudo apt-get install libfreeimage-dev