According to the SonntagsBlick, Swiss military intelligence intercepted a fax received by the Egyptian embassy in London supposedly confirming the existence of the detention centres.

The message was picked up by the secret service's Onyx satellite listening system on November 10, just three days after the Council of Europe launched its investigation into allegations that the CIA was running secret interrogation centres in Europe.

[...]

The Egyptian fax stated that 23 Iraqi and Afghan citizens had been transferred to a Romanian military base near the port of Constanza for interrogation purposes. It added that similar detention centres had been set up in Ukraine, Kosovo, Macedonia and Bulgaria.

The newspaper has reportedly been threatened with prosecution by the Swiss government for revealing intelligence information - which is strong evidence that the documents are genuine.

According to a quick translation on Daily Kos, the original article [German] identifies the Romanian base as a place called Mihail Kogalniceanu - a location which was already under suspicion.

The EU has already threatened suspension for any member-state found to have been host to part of the US's secret gulag, and if confirmed this may result in Romania's accession to the EU (currently scheduled for 2007) being delayed or even quashed.