I bought a digg's related .mobi a few days ago, then did some research (which i should of done before) .......
Found they had a TM and a note on their site saying words to the effect 'we don't mind people using digg terms, as long as the domain name doesn't have 'digg' in it anywhere' ..... Also, i remember reading digg.com are being forced into persueing TM Viols, because if they continue not too, their TM may default to voided on the grounds of not following up on any TMV's...
I'm sure if you search the Google you'll find what i'm refering too -
GD.