babayada,
Daniel Tammet DID memorize Pi. Check out this link from the magazine "Science":
http://sciencenow.sciencemag.org/cgi/content/full/2005/414/2
Although in the Brainman program he also demonstrates amazing calculation abilities, from what is reported here (and similar accounts elsewhere) I would say that he did not calculate Pi "on the go". In these reports it is implied that all 22,514 digits were already in his memory. However his memory is synesthetic, as you pointed out, which makes memorizing such a string of digits relatively easy for him.
I personally also find this whole topic fascinating. Especially the "why". There's a theory that these remarkable abilities are a result of damage to the left brain (such as a childhood epileptic seizure, as in the case of Daniel Tammet), resulting in an attempt by the right brain to compensate. In other words, everyone are in principle capable of these abilities, if they could switch one part of their brain "off" for awhile.
