Panoramic Software Inc. has just released Calc Pro for the iPhone and iPod Touch - I know a lot of people have been waiting for this.
Its very similar to the Windows Phone 7 version but contains a few improvements - most noticeably is the updated Date and Time calculator which has been redesigned and now shows the moon phase for the from and to dates.
It also of course makes use of the native Apple controls. It's normally $9.99 but its launching at 99 cents so its definitely worth grabbing now.
You can see it on the website here:
Calc Pro HD coming soon for the iPad