Ramifications of illegal content in transaction messages? Would it be possible to make using Bitcoin illegal by posting child porn or similar highly illegal content in the message of a transaction? There are methods for having 'thin clients' that don't download the entire transaction history - would these be immune to an attack like this? How difficult would it be to track the sender of such material?
Would it be possible to make using Bitcoin illegal by posting child porn or similar highly illegal content in the message of a transaction?


Bitcoin, as a decentralised provision network of sorts, may have the same protections as given to ISPs and content providers. So unless the government is gunning for Bitcoin anyway - I'd say no, Bitcoin can not be rendered illegal this way.


  There are methods for having 'thin clients' that don't download the entire transaction history - would these be immune to an attack like this? 


Obviously if you don't download the illegal content nor upload any intent to download specifically that type of content - you should be safe with a thin-client. But it limits the security of Bitcoin network considerably if only thin-clients were possible.


  How difficult would it be to track the sender of such material?


Not very difficult. Bitcoin is not an inherently anonymous protocol. If the communication between nodes is not encrypted with ephemeral TLS sessions, a mix service wouldn't provide any meaningful anonymity either.