Ubuntu suddenly logged out with no symptoms

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Ubuntu 11.04 randomly logs out in middle of use

Recently, I had a strange experiance. I boot up Ubuntu and login, start working, after 10-30 minutes (approx), it suddenly logged out, with no symption, very unregular. Then, I login again, it seems like the problem only occurred in the first login each day.

I've checked the syslog:

May 25 08:52:10 lenik-v anacron[1338]: Job `cron.daily' terminated
May 25 08:52:10 lenik-v anacron[1338]: Job `cron.weekly' started
May 25 08:52:10 lenik-v anacron[4861]: Updated timestamp for job `cron.weekly' to 2011-05-25
May 25 08:52:16 lenik-v anacron[1338]: Job `cron.weekly' terminated
May 25 08:52:16 lenik-v anacron[1338]: Normal exit (2 jobs run)
May 25 08:55:01 lenik-v CRON[4954]: (lenik) CMD (~/bin/update_pingdb)
May 25 08:58:13 lenik-v kernel: [ 1748.099194] warning: `VirtualBox' uses 32-bit capabilities (legacy support in use)
May 25 09:00:02 lenik-v CRON[5323]: (lenik) CMD (~/bin/update_pingdb)
May 25 09:05:02 lenik-v CRON[5835]: (lenik) CMD (~/bin/update_pingdb)
> May 25 09:05:14 lenik-v kernel: [ 2168.696447] show_signal_msg: 15 callbacks suppressed
> May 25 09:05:14 lenik-v kernel: [ 2168.696460] ibus-engine-sun[1491]: segfault at 19 ip 00000019 sp bfa91bcc error 4 in libsunpinyin.so.3.0[110000+45000]
> May 25 09:05:17 lenik-v kernel: [ 2172.106725] VirtualBox[5240]: segfault at a7309ff0 ip 009959c8 sp bfc23fe0 error 4 in libc-2.13.so[966000+15a000]
May 25 09:05:19 lenik-v NetworkManager[1220]: <info> (wlan0): device state change: 8 -> 3 (reason 38)
May 25 09:05:19 lenik-v NetworkManager[1220]: <info> (wlan0): deactivating device (reason: 38).
May 25 09:05:19 lenik-v avahi-daemon[1216]: Withdrawing address record for 192.168.1.12 on wlan0.
May 25 09:05:19 lenik-v avahi-daemon[1216]: Leaving mDNS multicast group on interface wlan0.IPv4 with address 192.168.1.12.
May 25 09:05:19 lenik-v avahi-daemon[1216]: Interface wlan0.IPv4 no longer relevant for mDNS.
May 25 09:05:19 lenik-v kernel: [ 2174.433437] wlan0: deauthenticating from 00:c0:02:f4:ce:aa by local choice (reason=3)

Here, I'm not sure if it's related to ibus-engine-sun or VirtualBox. ibus-engine-sun is a Chinese input method, and I run WinXP in VirtualBox, so both are not optional. But, both should be user application not kernel, isn't it?