I just got my HCHII 2 days ago and have noticed an odd sound: sometimes after coming to a stop (like pulling into my garage), I can hear what sounds like water splashing around in the back of the car.

It subsides quickly and is fairly faint. So far, I only notice it after I've turned off the car and when it's quiet around. Could it be the gas tank??

Yes, fuel sloshing around the sides of the tank.
I notice this more in my HCH probably because in auto-stop it is so deathly silent.
Sometimes I can even faintly hear what sounds to be a PC-style cooling fan back in the battery area as well.

Efficient drivers do it better.
1003 miles a tank personal record. 74MPG calculated. HCH1 CVT

Does the gas tank have a really thin lining or something? I don't think I've ever heard gas splashing around a tank in any other car before.

I can hear the fuel slosh in my 97 Civic. I think it's like Hot_Georgia said; the car is so quiet that it's just easier to hear. It's also possible the hybrid has less sound deadening material in that region either due to space constraints with the battery pack or in an effort to reduce weight. The old Integra Type-R had an unbaffled tank as a means of weight reduction. It's possible they have done the same with the hybrid to save a couple pounds.