Samsung has released the Chinese version of the i550, the SCH-i539. This appears to point towards that Sprint simply lost interest in the device, or that the device missed internal timeframes rather than a product-wide defect. Sprint has not issued any comment as to why the i550 was discontinued, and denied they ever announced it (despite it appearing in public trade shows for over six months as “a Sprint exclusive”).
This site has some information and pictures of the i539, as well as links to a petition to support Sprint returning the device to their lineup. SPI broke news that the i550 was cancelled, and despite many challenges to that statement, the device has been shown to not have been released.