Parenthesis don't show in built-in iPhone calculator app
The built-in calculator doesn't offer any visual clues to let you know you've opened a sub-group operation in an expression. The only thing that tips you off is you'll get a sub-result after you close the group show up in the display. For example, enter the following:

5 + ( 3 + 3 )


After you type the opening ( you don't see anything on the display. But when you press the closing ) the display shows you 6 to tip you off to the fact that the 3 + 3 operation was performed.

I prefer the TouchCalc app myself. It will show a small ( in the lower left corner of the display when you open a sub-expression grouping so you know you're actually perserving your order of operations as intended.