The upgrade, which requires shipping the camera to a Nikon service center, increases the camera’s buffer memory, allowing the D3 to capture longer continuous bursts before pausing to clear the card.
Shooting full-frame (what Nikon terms “FX”) with most file types, the expanded buffer capacity is substantial: the camera can capture more than twice as many fine-quality JPEGs (119, versus the default 52) or 14-bit NEF raw files (36 versus 16) when shooting with continuous drive. Details on the upgrade, including a table detailing file capacity improvements, can be found on Nikon’s European support site.
Details on the service for U.S. customers have yet to be released, but the upgrade won’t come cheap in any event: expect to pay about $500 (plus shipping costs and tax) for the expanded capacity.
UPDATE: Information from the U.S. support site is now available.