Originally we were only going to support the latest (1.6), but then the new 1.8 version was released and I started adding support to that as well. Then we noticed that dreadfully slow/out of date RHEL and derivatives still have 1.4 versions of Cairo floating around (yuck). So the extension will support 1.4+ However the cairo docs are incorrect in a couple of places regarding new symbols. After some header diffing here is a complete list for 1.8 and 1.6 (to make life easier for anyone else writing language wrappers).
Her list includes forty new symbols for the 1.8 release of the extension and twenty-two for version 1.6.