Found the changes for B3 here:

http://www.intel.com/Assets/PDF/specupdate/320839.pdf

If you have B2 and are curious what changed in B3, then it looks like the following were either fixed or do not apply to B3:

3319486, 3319733: DEVCON2[3:0] and CTOCTRL not accurately set the
completion timeout value.

3319609: Boot-dependent QPI CRC errors

3319688: Some Gen2 endpoints will not complete the training in Gen2 mode.

3319737: MIERRCNT does not properly count persistent SMBus retry failures
and MINNERRST doesn't log the errors (HEDT only)

3319811: EOI to the I/OxAPIC can be blocked

3319755: GTIME upper 32 bits can not be read or written.

3319791: Intel® QuickPath Interconnect (QPI) L0s and L1 Power Management
Link State Fails

3319795:PCI Express* 2.0 L0s Link Recovery Fails

3319874: IOH May Falsely Assert THERMTRIP_N Signal After a Reset Event

3319408: VT-d queue-based invalidation is enabled only when enabled on both
channels (WS only)

3319432: VTd does not support the draining of compatibility-format interrupts
(WS only)

3319452: Hardware applies HPA_LIMIT to upstream memory request when VTd
is disabled (WS only)

3319630: VT-d: Memory read request with AT=11 results in malformed TLP

3319532: VT-d: IOTLB Domain-page-selective invalidation not working
correctly.

3319847: The IOH Ignores Snoop Behavior Bit in VT-d Page Table.

3319819: VT-d reports an Invalidation Queue Error when the Queue size is 7
(HEDT only)
Now I want to know if/when the DX58SO desktop board will include this. The latest revision E29331-503 does not mention this new B3 X58.