No problem.
When I use separate PINs (5, 3, 6, 9) I get the following two text messages in every loop:
Checking NFC...
Found 0 tags
The first line is generated by my code and the second one by the NFC library.
When I put one RFID tag on the reader, I get the message "Found 1 Tag" and then my code continues with the Ethernet/http connection to my server without any issue. Finally my application goes back to the NFC reading loop after closing the connection.
When I reuse SPI PINs (5,11,12,13) I only see one text line in every loop (no text line from the NFC library); no tag is detected by the NFC reader:
Checking NFC...
I downloaded NFC library from the wiki of SeeedStudio: NFC Shield | Seeed Studio Wiki
http://www.seeedstudio.com/wiki/images/2/25/PN532_SPI_V1.zip