No cable yet, as Apple will likely release the first one, as the iPod also need to support synching via TB.
Also the speed of the transfer is limited to the speed of the storage device in the iPod, if it is a HDD in an iPod Classic, the speed provided by USB 2.0 will suffice, as the internal HDD is already a slow HDD. If it is an iPod touch or nano or shuffle, the flash memory used in it will not be that fast, as music playback does not need fast read access, thus slower flash memory will have been used in those iPods. Again, USB 2.0 will suffice those speeds.