I'm running vmware workstation on a dual core AMD 64 CPU, and am getting 
a crash on 4.0_BETA2 kernel compiled from the auto build site dated 
200703280002Z.  This is fairly reproducable since building a -current or 
4.x branch kernel will trigger it pretty much every time.

Here's what I see on the screen just prior to the machine rebooting:

uvm_fault(0xffffffff809e3b40, 0xffff8001110d9000, 1) -> e
fatal page fault in supervisor mode
trap type 6 code 0 rip ffffffff80461070 cs 8 rflags 10202 cr2 
ffff8001110d9000
cpl 5 rsp ffff800013a6d6f0
panic: trap
syncing disks...

<and the machine reboots.>

vmware could be the culprit here I suppose as the 64-bit support is new 
and experimental.  I have two virtual machines (one will become -current 
if it compiles a kernel ever...) and the other will remain 4.x.  Both 
will be used for compile and integration testing.

The source tree is on an NFS partition (which happens to be on another 
vmware virtual machine, running i386 4.0_BETA2 kernel) so nfs client 
could also be the problem.

Unfortunately it does not drop into DDB so I can't trace it further at 
this point.