Both the iPhone 4 and the iPad can connect to the faster 802.11n networks, but it is not as clear cut as that.
Whilst the iPad can connect to a 5GHz 802.11n wireless network the iPhone 4 can only connect to an 802.11n wireless networking using 2.4GHz.
Whilst that is much faster than connecting to an 802.11g network one reason for having a 5GHz is to avoid the often crowded 2.4GHz spectrum as the rest of the world has discovered the advantages of wireless networking.
So if you if you only have a single 802.11n wireless network you will need to ensure it is either running at 2.4GHz or add a new network just for the iPhone, which is a little bit over the top.
I can see myself adding a 2.4GHz 802.11n network for my new iPhone 4 in the main so I can stream video over my network using the AirVideo App.