Hi everybody, I ran into many roadbumps installing this, hopefully these tips will help somebody else.
I installed it on a CentOS 5.x system running PHP 5.3.x managed by Virtualmin/Webmin. Using SSH, I installed the following packages via yum:
yum install php53-devel geoip geoip-devel gcc.
Basically I ran into a bunch of random errors that were resolved by installing whatever package was required, AND their equivalent -devel companion. Then I ran:
pecl install geoip
Once that installed, I had to edit /etc/php.ini. Search for "extension=" and add the line "extention=geoip". If you have virtual hosts setup in your hosting environment that each have their own php.ini file, you'll need to add that line to each file.