Is there an iOS-compatible Objective-C-port of the tcpdump C-sources or a working tcpdumpbinary for iOS? And if so, does it capture the traffic from all network devices? Or does it capture only an Ethernet device? Is there libpcap for iOS? I am aware, that this requires a jailbroken Phone.
Yes to both:
http://planet-iphones.com/cydia/id/tcpdump
http://planet-iphones.com/cydia/id/libpcap
However I don't understand why you want an Objective-C port?