I am currently doing some trademark related research, and just for fun I queried the database of the DPMA (German trademark and patent office) for "netfilter".
To my big surprise, you can find this record, indicating that Deutsche Telekom AG has applied for a trademark on the word "NetFilter" in July 2006.
I find that quite outrageous, as the netfilter project is using the name since about 1999, i.e. 7 years earlier. To our luck, the trademark office refused the application based on the generic nature of the name, i.e. "netfilter" being too generic for anyone obtaining a trademark on it - at least in Germany, under German laws.