I also noticed a tingle when touching the case of my new iPad while charging this morning. Reversing the non-polarized plug of the 12w USB power adapter was effective in eliminating the tingle.
I measured the AC voltage from the case to ground with a digital multimeter and found about 4.5 volts and 2 volts depending on the polarity of the AC plug. Unfortunately, the AC current measuring range on my meter didn't seem to be working, but when touching the case with one hand and ground with the other, the voltage dropped about in half. Then I measured the resistance between my two dry hands and found about 1.3 megohms. Based on this, the adapter internal impedance is probably around this same value, so the leakage current with wet skin might be up to about 5 microamps, which should be safe. BTW, the tingling sensation is a lot more obvious when touching both ground and the case.
It's likely that there is some capacitive coupling between the AC line and ground (RFI filter capacitors and/or capacitance between isolation transformer primary & secondary windings) that causes this. It's very common with 2-wire appliances.
All in all, probably not a big deal, but try reversing the AC plug and see if the tingling is reduced. Too bad Apple doesn't supply an adapter with a 3-prong AC plug, which would ground the case and eliminate the issue.