I too experience the spontaneous reboots on a mac pro. I am able to reproduce it on my mac pro running 10.5.3. If I put my mac to sleep then wake it and attempt to power on any 64-bit windows guest I get an spontaneous reboot. 32-bit guests seem to work fine.
The only things I can find in the logs are:
Jun 3 18:36:42 Opus /System/Library/CoreServices/backupd
212: Forcing deep traversal on source: "Macintosh HD" (mount: '/' fsUUID: 5DDFC7BF-4D3B-32AC-B8A6-2C6765B391CB eventDBUUID: 940FB2B6-6E6A-440E-89F2-245A579E9746), {1, 1, 1, 0}
Jun 3 18:36:42 Opus /System/Library/CoreServices/backupd
212: Node requires deep traversal:/ reason:kFSEDBEventFlagMustScanSubDirs|kFSEDBEventFlagReasonForceDeepBackup|
Jun 3 18:36:50 Opus kernel[0]: vmmon: offset 0: 44
Jun 3 18:36:50 Opus kernel[0]: vmmon: offset 1: 12
Jun 3 18:36:50 Opus kernel[0]: vmmon: offset 2: 24
Jun 3 18:36:50 Opus kernel[0]: vmmon: offset 3: 28
Jun 3 18:36:50 Opus kernel[0]: vmmon: offset 4: 40
Jun 3 18:36:50 Opus kernel[0]: vmmon: PTSC: initialized at 2793448000 Hz using reference clock
Jun 3 18:36:50 Opus kernel[0]: vmmon: INTELRDMvvvSmmmmRmom(on: 0xon: INTELRDMSR(0x8b) = 0x0000n:INTEL 060b00IR8000000
Jun 3 18:36:50 Opus kernel[0]: DMSR(Nvb0)mTx8b) = 0x0000060b00000000
Jun 3 18:36:50 Opus kernel[0]: mon: INTELRDMSR(0x17) = 0x0E0 vmmon: INTELRDMSR(0x17) = 0x0018000088640720
Jun 3 18:36:50 Opus kernel[0]: 18000088640720
Jun 3 18:36:50 Opus kernel[0]: = 0x0000060b00000000
Jun 3 18:36:50 Opus kernel[0]: LRDMSR(0x8b) = 0x0000060b00000000
Jun 3 18:36:50 Opus kernel[0]: vmmon: INTELRDMSR(0x17) = 0x0018000088640720
Jun 3 18:36:50: --- last message repeated 1 time ---
Jun 3 18:36:50 Opus kernel[0]: vmmon: Warning: XXXMACOS: Implement ioctl APIC_BASE
Jun 3 18:36:51 Opus kernel[0]: vmci: Open socket 0x6fea444.
Jun 3 18:36:51 Opus kernel[0]: vmmon: Cycles 0x00000000000000E7
Jun 3 18:36:51 Opus kernel[0]: vmnet: VNetUserIf_Create: created userIf at 0x85bbe00.
Jun 3 18:36:51 Opus kernel[0]: vmnet: VMNetConnect: returning port 0x85bbe00
Jun 3 18:36:51 Opus kernel[0]: vmnet: bridge-en0: enabled promiscuous mode
Jun 3 18:36:51 Opus kernel[0]: vmnet: VMNET_SO_BINDTOHUB: port: paddr 00:50:56:f5:7c:47
Jun 3 18:36:51 Opus kernel[0]: vmnet: Hub 0
Jun 3 18:36:51 Opus kernel[0]: vmnet: Port 0
Jun 3 18:36:51 Opus kernel[0]: vmnet: Port 1
Jun 3 18:36:51 Opus kernel[0]: vmnet: VNetUserIf_Create: created userIf at 0x85bba00.
Jun 3 18:36:51 Opus kernel[0]: vmnet: VMNetConnect: returning port 0x85bba00
Jun 3 18:36:51 Opus kernel[0]: vmnet: VMNET_SO_BINDTOHUB: port: paddr 00:50:56:f8:42:1f
Jun 3 18:36:51 Opus kernel[0]: vmnet: Hub 0
Jun 3 18:36:51 Opus kernel[0]: vmnet: Port 0
Jun 3 18:36:51 Opus kernel[0]: vmnet: Port 1
Jun 3 18:36:51 Opus kernel[0]: vmnet: Port 2
Jun 3 18:36:51 Opus kernel[0]: vmnet: VNetUserIfFree: freeing userIf at 0x85bba00.
Jun 3 18:36:51 Opus kernel[0]: vmmon: host clock rate change request 0 -> 19
Jun 3 18:36:51 Opus kernel[0]: vmmon: host clock rate change request 19 -> 83
<---- here is where the reboot happens im assuming.
Jun 3 18:37:10 localhost com.apple.launchctl.System[2]: fsck_hfs: Volume is journaled. No checking performed.
Jun 3 18:37:10 localhost com.apple.launchctl.System[2]: fsck_hfs: Use the -f option to force checking.
Directory service logs:
2008-05-30 19:29:59 EDT - T
0xA01ADFA0 - Improper shutdown detected
2008-06-01 16:48:00 EDT - T
0xA01ADFA0 - Improper shutdown detected
2008-06-02 19:11:02 EDT - T
0xA01ADFA0 - Improper shutdown detected
2008-06-03 18:32:36 EDT - T
0xA01ADFA0 - Improper shutdown detected
2008-06-03 18:37:14 EDT - T
0xA01ADFA0 - Improper shutdown detected