With version 5, IBM has incorporated a number of things in use in the Linux world. All version 5 AIX releases have the L suffix, with L standing for, you guessed it, Linux. What this means is a number of weaknesses in AIX have been addressed. One of the most annoying things for me was the lack of a /dev/random. You always had to install prngd (Pseudo random number generator daemon) on AIX. No longer. AIX (current version is 5.3L) now has /dev/random, so openssl installs cleanly.
RPM is installed by default, so two package managers are in widespread use on current AIX servers, installp and RPM. GCC now works very well on AIX, allowing the building of a lot of the gnu software library. AIX has become a lot more flexible because of these changes. I seem to see a lot more AIX job openings nowadays as well. A lot of companies have apparently been sold on IBM's Linux affinity.
AIX runs on IBM's Power series CPUs.