The case will be filed and the police will subpoena his IP address from Twitter, who will comply with the warrant. Once they have the IP address they will subpoena his ISP for his personal details.
If he is identified from all of this, he will be arrested... but the case will fail on lack of evidence that he actually caused any obstruction of justice in any specific instance.
EDIT: and a hundred more PigSpotters will spring up on social networking sites due to the publicity that the police have brought to this case.
Yes, you are totally correct. The only way in which the PS will be traced will be when the police gets a court order and serve it on Twitter, who will in turn supply his IP address. However, from my own experience here goes:
Twitter is in the US and will not act if contacted by the SAPS. They will require a court order. A South African court order wont do either, so here is the process:
1. Once prosecution decides to proceed, the I/O would need to finish the investigation by obtaining the relevant affidavits and evidence. (Could take a month to a year)
2. The I/O will need to apply for a court order for the info from Twitter. (Takes about a day)
3. The court order will now go to Interpol, who will in turn refer it to US law enforcement agencies. Once there a court will review the matter, and if in agreement issue a court order there. That order will be served on Twitter. (This could take 3 years or longer. I left the police in 2005. I sent 2 orders to ebay in 2002. Last year the docket was still closed and nothing came back as yet. The fastest one I got back from the US was in 4 years.)
4. Twitter will now send documents to US law enforcement who will send it to Interpol, who in turn will send it back to SA. (Could take about a year if the documents dont get missing somewhere along the way)
5. The I/O now needs to re-apply for court orders on the telecommunications department. (Another day).
6. Telecommunications now have 30 days to provide the info on the phone number from where the dialup was made to the Internet.
I bet you all, this case will not be completed - with proper evidence - within at least the next 5-8 years.