I updated from 3.3.13 to 3.5.11 (and later also tried 3.4.14), and whenever I try to connect, via smbclient for example, I get a core dump just after entering my password.

3.3 works just fine, so I have reverted to that.
Happy to provide more info though, if you can tell me what you want.

Example log info:

[2011/09/08 22:29:41,  0] lib/util_tdb.c:tdb_log(682)
 tdb(/var/db/samba/gencache.tdb): tdb_reopen: file dev/inode has changed!
[2011/09/08 22:29:41,  0] lib/util.c:reinit_after_fork(1054)
 tdb_reopen_all failed.
[2011/09/08 22:29:41,  0] smbd/server.c:open_sockets_smbd(773)
 reinit_after_fork() failed
[2011/09/08 22:29:41,  0] lib/util.c:smb_panic(1673)
 PANIC (pid 86439): reinit_after_fork() failed
[2011/09/08 22:29:41,  0] lib/util.c:log_stack_trace(1777)
 BACKTRACE: 0 stack frames:
[2011/09/08 22:29:41,  0] lib/fault.c:dump_core(231)
 dumping core in /var/log/samba/cores/smbd

And the corresponding smbclient session:

smbclient -d4 '\\myserver\myshare
lp_load_ex: refreshing parameters
Initialising global parameters
max_open_files: increasing sysctl_max (11095) to minimum Windows limit (16384)
rlimit_max: increasing rlimit_max (11095) to minimum Windows limit (16384)
params.c:pm_process() - Processing configuration file "/usr/local/etc/smb.conf"
Processing section "[global]"
doing parameter workgroup = WORKGROUP
doing parameter server string = 
doing parameter security = share
doing parameter hosts allow = 192.168.1. 127.
doing parameter printing = bsd
doing parameter load printers = no
doing parameter guest account = smbguest
doing parameter log file = /var/log/samba/log.%m
doing parameter max log size = 50
doing parameter dns proxy = no
pm_process() returned Yes
added interface nfe0 ip=192.168.1.200 bcast=192.168.1.255 netmask=255.255.255.0
Client started (version 3.5.11).
Enter myuser's password: 
tdb(/var/db/samba/gencache.tdb): tdb_open_ex: could not open file /var/db/samba/gencache.tdb: Permission denied
resolve_lmhosts: Attempting lmhosts lookup for name myserver<0x20>
startlmhosts: Can't open lmhosts file /usr/local/etc/lmhosts. Error was No such file or directory
resolve_wins: Attempting wins lookup for name myserver<0x20>
resolve_wins: WINS server resolution selected and no WINS servers listed.
resolve_hosts: Attempting host lookup for name myserver<0x20>
Connecting to 192.168.1.200 at port 445
 session request ok
protocol negotiation failed: NT_STATUS_PIPE_BROKEN