Hardware-wise, XC is HC. Wikipedia has more details. There are some reports of working cards here. Speed-wise, Nokia disabled high speed (either 24 or 48MHz) support in the driver as it caused problems with some cards (and I was also too chicken to re-enable for everyone in the CSSU), but there are simpe patches to re-enable it around for those who need the extra speed and know what they are doing.
That's because the SD driver runs on the embedded controller inside the adaptor and can't be updated :-(