TechCrunch generally disclose when Crunchfund invest (albeit they didn't do so effectively with Orchestra/Mailbox). Having said that I don't feel that they need to disclose that a former founder & now part-time Writer, has an investment in a VC firm - which he has no control over - and they have made an investment in a startup which, he may not even be aware about.

Micheal Arrington is an LP in a16z, Benchmark, Lerer Ventures, SV Angel as well as being a General Partner at Crunchfund[1]. I think the disclosure of Crunchfund is enough otherwise, on every post there will be a disclosure stating Michael Arrington has directly/indirectly invested in a startup they are covering with/without his own knowledge. Don't forget this is the same, Michael Arrington who stopped investing when he founded TechCrunch and the ones he had invested in - there was a clear disclosure and he never personally covered them.

[1] http://www.crunchbase.com/person/michael-arrington