It is not a supported platform by any vendor, however binaries exist for a Java compatible runtime and development kit known as the IBM Development Kit, available for free. This has probably not been available since 2005, when you posted this question, but for what it's worth there is also a pre-release program for Java 6 on the following website:
PPC/Linux is supported by IBM on PSeries, and Sun does not support Java compatible implementations directly, but if the code is written strictly to Java 2 SE 5.0 specification, it will run on a compatible implementation such as Blackdown, IBM, Apple's varient, or FreeBSD's. The only PPC compatible implementations are IBM's and GNU's, but only IBM's will run applets.
J2EE is part of IBM WebSphere, which is not free afaik. You should be able to run Glassfish with the IBM Development Kit using the generic Java JAR, but cross your fingers.