I have not heard of anyone owning the wireless charging shell for the 720 - it's not in stock anywhere and hasn't been. Amazon UK seems to expects a 1-3 month delivery time.
Sometimes it seems hard to believe that Nokia is trying the least to be a serious company, even remotely. The 720 has not even been released in Scandinavia yet (not available before the end of this month - maybe). This is a mid-range phone. Yes, there's demand, but the demand is still completely dwarfed compared to the HTC One series (V, X, etc.) alone combined.
So until now, it has taken them 4 months from presentation to release of a mid range device. Approximately the same was true for the 620. So the fact that the 720 wireless charging shell is not available yet is just the icing to the cake. It's no surprise that the market share of WP8 can't possibly sky-rocket when the turds at Nokia has lost their ability to produce and introduce phones like in the old days.