As you can see, I'm running FC6 with kernel 2.6.18-1.2868.fc6 (the latest version update from YUM repos). Last night, went to bed leaving system running (BitTorrent), this morning got up and found it crashed with the following messages in /var/log/messages:
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Dec 25 01:55:08 localhost kernel: SELinux: initialized (dev hdd, type iso9660), uses genfs_contexts
Dec 25 04:08:29 localhost kernel: BUG: unable to handle kernel NULL pointer dereference at virtual address 00000000
Dec 25 04:08:29 localhost kernel: printing eip:
Dec 25 04:08:29 localhost kernel: 00000000
Dec 25 04:08:29 localhost kernel: *pde = 26e25067
Dec 25 04:08:29 localhost kernel: Oops: 0000 [#1]
Dec 25 04:08:29 localhost kernel: SMP
Dec 25 04:08:29 localhost kernel: last sysfs file: /block/hdd/size
Dec 25 04:08:29 localhost kernel: Modules linked in: nls_utf8 ntfs(U) autofs4 sunrpc ip_conntrack_netbios_ns ipt_REJECT xt_state ip_conntrack nfnetlink iptable_filter ip_tables ip6t_REJECT xt_tcpudp ip6table_filter ip6_tables x_tables dm_mirror dm_multipath dm_mod video sbs i2c_ec button battery asus_acpi ac ipv6 parport_pc lp parport tuner snd_intel8x0 snd_ac97_codec snd_ac97_bus snd_seq_dummy snd_seq_oss snd_seq_midi_event snd_seq saa7134 snd_seq_device video_buf compat_ioctl32 ir_kbd_i2c ir_common forcedeth floppy videodev nvidia(U) v4l1_compat snd_pcm_oss snd_mixer_oss v4l2_common snd_pcm snd_timer snd soundcore snd_page_alloc ide_cd cdrom i2c_nforce2 i2c_core serio_raw pcspkr sata_nv libata sd_mod scsi_mod ext3 jbd ehci_hcd ohci_hcd uhci_hcd
Dec 25 04:08:29 localhost kernel: CPU: 0
Dec 25 04:08:29 localhost kernel: EIP: 0060:[<00000000>] Tainted: P VLI
Dec 25 04:08:29 localhost kernel: EFLAGS: 00210217 (2.6.18-1.2868.fc6 #1)
Dec 25 04:08:29 localhost kernel: EIP is at _stext+0x3fbffd6c/0x3c
Dec 25 04:08:29 localhost kernel: eax: 00000000 ebx: 00000000 ecx: 00000000 edx: 00000000
Dec 25 04:08:29 localhost kernel: esi: e8b026c0 edi: bf9b8864 ebp: e71b8000 esp: e71b8fa4
Dec 25 04:08:29 localhost kernel: ds: 007b es: 007b ss: 0068
Dec 25 04:08:29 localhost kernel: Process knotify (pid: 22177, ti=e71b8000 task=e6be3690 task.ti=e71b8000)
Dec 25 04:08:29 localhost kernel: Stack: 00000019 00000000 00000004 bf9b8864 0884abd0 c0404013 00000004 0000541b
Dec 25 04:08:29 localhost kernel: bf9b8864 bf9b8864 0884abd0 bf9b8818 ffffffda 0000007b 0000007b 00000008
Dec 25 04:08:29 localhost kernel: 00180011 00000073 00200216 bf9b87f4 0000007b 00000000 00000000
Dec 25 04:08:29 localhost kernel: Call Trace:
Dec 25 04:08:29 localhost kernel: Inexact backtrace:
Dec 25 04:08:29 localhost kernel: [<c0404013>] syscall_call+0x7/0xb
Dec 25 04:08:29 localhost kernel: =======================
Dec 25 04:08:29 localhost kernel: Code: Bad EIP value.
Dec 25 04:08:29 localhost kernel: EIP: [<00000000>] _stext+0x3fbffd6c/0x3c SS:ESP 0068:e71b8fa4
Dec 25 08:09:34 localhost syslogd 1.4.1: restart.
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The system was in the state of not responding to any input from the keyboard, let alone the mouse... the screen was black (as if due to the power saving). The restart you can see was the result of me hitting the reset button (go figure!). As far as I can see, most of the messages are CPU related, since CPU registers are among everything else, and CPU is mentioned itself.

Now, this is the second time I'm experiencing kernel crash in the last two months. The first might have been the memory issue (though, I didn't have the time to look into it)... and it was another kernel version.

Should I be worried and expect more of messages like this?

BTW: I have 2*Kingston 512 DDR 333MHz memory modules, and an AMD Athlon 2000+ processor, in case it matters.

its your cdrom, unplug it.

these things are very hard to figure out.
it seems the kernel faulted (no page found) on a register read.
it is possible for it to be hardware but i don't know how likely.
no way i see how to say it was the disk drive so that seems like a leap.
sometimes it has to do with modules unloading when they should not especially when the machine is idle.
sometimes it's a bug in the code: OOPS

all i can tell you more and i might be out to lunch on this
the process knotify caused the problem which i think is a KDE desktop thing
(you use KDE ?)
first thing to do is go into the KDE thingy and see if you can turn knotify off.
might not be able to.
it did a syscall to the kernel -- when a userspace program calls a syscall, it calls int 0x80 with the number of the syscall stored in the eax register.
Then the kernel uses the function system_call() that is implemented on x86
kernel tree in arch/i386/kernel/entry.S to call the right syscall
requested by the user.
your eax register was NULL
might be hardware interupt related or a bad driver or just some bad kernel code i just don't see any way to know. i never trust those jacked redhat kernels and i would try compiling my own vanilla kernel like it comes from the developers. I have read some developer boards where the redhat people are kernel hacking in full clueless mode.