key_buffer_size=8384512
read_buffer_size=131072
max_used_connections=8
max_threads=151
threads_connected=3
It is possible that mysqld could use up to 
key_buffer_size + (read_buffer_size + sort_buffer_size)*max_threads = 337724 K
bytes of memory
Hope that's ok; if not, decrease some variables in the equation.

thd: 0xb2217ca8
Attempting backtrace. You can use the following information to find out
where mysqld died. If you see no messages after this, something went
terribly wrong...
stack_bottom = 0xb48e43b8 thread_stack 0x30000
/usr/local/mysql/bin/mysqld(my_print_stacktrace+0x21)[0x84d6981]
/usr/local/mysql/bin/mysqld(handle_segfault+0x381)[0x8200291]
[0xffffe400]
/usr/local/mysql/bin/mysqld(_Z8filesortP3THDP8st_tableP13st_sort_fieldjP10SQL_SELECTybPy+0x11db)[0x82ebc4b]
/usr/local/mysql/bin/mysqld[0x8271bc8]
/usr/local/mysql/bin/mysqld(_ZN4JOIN4execEv+0x8e7)[0x827c447]
/usr/local/mysql/bin/mysqld(_Z12mysql_selectP3THDPPP4ItemP10TABLE_LISTjR4ListIS1_ES2_jP8st_orderSB_S2_SB_yP13select_resultP18st_select_lex_unitP13st_select_lex+0xdb)[0x827865b]
/usr/local/mysql/bin/mysqld(_Z13handle_selectP3THDP6st_lexP13select_resultm+0x15e)[0x827e10e]
/usr/local/mysql/bin/mysqld[0x820d1d0]
/usr/local/mysql/bin/mysqld(_Z21mysql_execute_commandP3THD+0x32b4)[0x8212f04]
/usr/local/mysql/bin/mysqld(_Z11mysql_parseP3THDPKcjPS2_+0x340)[0x8217430]
/usr/local/mysql/bin/mysqld(_Z16dispatch_command19enum_server_commandP3THDPcj+0x11e0)[0x8218620]
/usr/local/mysql/bin/mysqld(_Z10do_commandP3THD+0xe0)[0x8218ea0]
/usr/local/mysql/bin/mysqld(handle_one_connection+0x233)[0x82099a3]
/lib/i686/cmov/libpthread.so.0[0xb7efa4c0]
/lib/i686/cmov/libc.so.6(clone+0x5e)[0xb7e036de]
Trying to get some variables.
Some pointers may be invalid and cause the dump to abort...
thd->query at 0x8c8df18 = SELECT userid, usergroupid, displaygroupid, username, posts
						FROM user AS user
						WHERE posts > '0' 
						ORDER BY posts DESC
						LIMIT 0, 5
thd->thread_id=8646
thd->killed=NOT_KILLED
The manual page at http://dev.mysql.com/doc/mysql/en/crashing.html contains
information that should help you find out what is causing the crash.
091221 13:10:27 mysqld_safe Number of processes running now: 0
091221 13:10:27 mysqld_safe mysqld restarted
091221 13:10:27  InnoDB: Started; log sequence number 0 86140
091221 13:10:27 [Note] Event Scheduler: Loaded 0 events
091221 13:10:27 [Note] /usr/local/mysql/bin/mysqld: ready for connections.
Version: '5.1.34'  socket: '/tmp/mysql.sock'  port: 3306  MySQL Community Server (GPL)