with mustang b29 + CMS collector, atg failed immediately.
The failure happened across platform, with both server and client vm.
# uname -a
SunOS jtg-e450 5.10 Generic sun4u sparc SUNW,Ultra-4
# /usr/j2se/bin/java -version
java version "1.6.0-ea"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.6.0-ea-b29)
Java HotSpot(TM) Client VM (build 1.6.0-ea-b29, mixed mode, sharing)

Flags used:
-XX:+UseConcMarkSweepGC -XX:+CMSClassUnloadingEnabled -XX:+CMSPermGenSweepingEnabled

Stack trace:
[1] ___nanosleep(0xf927eef0, 0xf927eee8, 0x10, 0x0, 0xff11bbe5,
0x6e2), at 0xff33c7dc
  [2] sleep(0x64, 0xf927ef58, 0x0, 0x0, 0x0, 0x0), at 0xff32faa0
  [3] os::message_box(0xff0d2641, 0xff11baf8, 0x4e, 0x64, 0xf927ef68,
0xf927ef58), at 0xfefe7460
  [4] VMError::show_message_box(0xf927f128, 0xff11baf8, 0x7d0, 0x4400,
0x94030, 0x135c), at 0xff062034
  [5] VMError::report_and_die(0xf927f128, 0x37f8, 0x5080, 0x6, 0x45fc,
0x0), at 0xff061358
  [6] JVM_handle_solaris_signal(0xb, 0xf927f5a8, 0xf927f2f0, 0x3c00,
0x3ed8, 0xe7f38), at 0xfede096c
  [7] __sighndlr(0xb, 0xf927f5a8, 0xf927f2f0, 0xfede006c, 0x0, 0x1), at
0xff33c52c
  [8] call_user_handler(0xb, 0xffbffeff, 0xc, 0x0, 0xfe820c00,
0xf927f2f0), at 0xff331998
  [9] CMSCollector::refProcessingWork(0xc7948, 0x1000, 0x44c4, 0x0,
0x6ea, 0xe83dc), at 0xfee60678
  [10] CMSCollector::checkpointRootsFinalWork(0xc7948, 0x4000, 0x0,
0x44c4, 0x0, 0xff107aa0), at 0xfee5e8ac
  [11] CMSCollector::checkpointRootsFinal(0xc7948, 0x1, 0x0, 0x0,
0xb6920, 0x297a88), at 0xfee5e654
  [12] CMSCollector::do_CMS_operation(0xc7948, 0x1, 0xe96c8, 0x2948e8,
0xff108944, 0xff108724), at 0xfee61854
  [13] CMSCollector::stop_world_and_do(0xc7948, 0x1, 0x354f0, 0x4928,
0x4bb4, 0x4800), at 0xfee61ae0
  [14] CMSCollector::collect_in_background(0xc7948, 0xff106908,
0xff0f6000, 0xc7bb8, 0xff10de1c, 0x1), at 0xfee5a0d8
  [15] ConcurrentMarkSweepThread::run(0x1, 0x7d0, 0xe7f38, 0x0,
0xff10de1c, 0xff0f6000), at 0xfee67048
  [16] _start(0xe7f38, 0x29a8, 0x5000, 0x0, 0xff0b8b80, 0xff0f6000), at
0xfefe57ac

The failue is gone if the flags -XX:+CMSClassUnloadingEnabled -XX:+CMSPermGenSweepingEnabled are removed.
The failure does NOT happen with ParallelGC and SerialGC.
###@###.### 2005-03-24 19:05:07 GMT

S1AppServer also fails with CMS + -XX:+CMSClassUnloadingEnabled -XX:+CMSPermGenSweepingEnabled
###@###.### 2005-03-24 19:08:25 GMT