Jonathan Brossard gave a great talk about this at defcon last year. Around the 2:30 mark in the video he talks a bit about the idea of China backdooring hardware.

http://www.youtube.com/watch?v=yRxDvkKBMTc

http://www.slideshare.net/endrazine/defcon-hardware-backdoor...

http://www.scribd.com/doc/101181012/Rakshasa-Whitepaper