This is being discussed over at Technet at the moment, but it would be interesting to know if anyone here is having the same issue:
http://social.technet.microsoft.com/Forums/en-US/smallbusinessserver/thread/16cac6e4-ffb5-4920-b48e-0f484ae8c8ff

Basically the NTDLL.dll is crashing svchost.exe which in turn causes the RPC service to terminate unexpectantly.  As a result of the unexpected termination the server reboots itself.

Error message about rebooting:
The process C:\Windows\system32\services.exe (SERVERNAME) has initiated the restart of computer <SERVERNAME> on behalf of user NT AUTHORITY\SYSTEM for the following reason: No title for this reason could be found
 Reason Code: 0x30006
 Shutdown Type: restart
 Comment: Windows must now restart because the Remote Procedure Call (RPC) service terminated unexpectedly

Error message about NTDLL.dll
Faulting application name: svchost.exe, version: 6.1.7600.16385, time stamp: 0x4a5bc3c1
Faulting module name: ntdll.dll, version: 6.1.7600.16385, time stamp: 0x4a5be02b
Exception code: 0xc0000005
Fault offset: 0x0000000000024afd
Faulting process id: 0x388
Faulting application start time: 0x01cc2597bdd008de
Faulting application path: C:\Windows\system32\svchost.exe
Faulting module path: C:\Windows\SYSTEM32\ntdll.dll
Report Id: 490ea8e1-97a3-11e0-bc70-000c294813a3

Any ideas?  Thinking it is a bug with SBS2011.

NOTE: SBS2011 is running on a VM, completely new build. only apps that are running are:
ESET NOD32 Mail Security
StorageCraft Shadow Protect SBS

And then the usual Exchange 2010, DNS, DHCP, etc

Disabling or uninstalling the above apps makes no difference. 

It sucks cause it is a live server, thankfully mail isnt a huge requirement for them.