Using merlin-beta3 release (1.4) latest build 80 :
 took a simple "Hello World" program and ran it using hprof.

prithvi 237 =>./bin/java -Xrunhprof:heap=dump test
Hello World
Dumping Java heap ... done.
prithvi 246 =>grep ARR java.hprof.txt | grep 'Unknown'
ARR ffe78 (sz=424, trace=1, nelems=102, elem type=<Unknown>@ffe58)
ARR ffeb8 (sz=24, trace=1, nelems=2, elem type=<Unknown>@ffe98)
ARR fff18 (sz=24, trace=1, nelems=2, elem type=<Unknown>@ffe98)
ARR fff78 (sz=24, trace=1, nelems=2, elem type=<Unknown>@ffe98)
....................

Not that every array object looks like this, but there are many such object array records in the output for which we have not seen the element class.

I performed the same test using JDK 1.3.1 (build 24), but could n't find any object arrays in the output with "Unknown" element type.

Please refer to Incident Manager Bug Report # 132318.

The only JVMPI bug I could find related to array objects is 4330936 which is fixed in merlin-beta release.