I tried with three different pairs of cans and not a problem, the mini plug goes until the end and they are recognized by my iThingie as headphones.
I used a standard iPod white ones (not great sound), Akai semipro cans (the best sound) and even a 3connector official Samsung Wave headset with mic:
It's working! Lucky me.
MacGliss @ iPad first gen. 64GB WiFi 3G.