People picker field has
Display Name
AccountID
Account Type
in it....
you can set default values for each of these fields....
1) Add a people picker on infopath form
2) Take properties of any of above (sub fields) [I have picked Account ID in my example]
You can take properties by selecting AccountID in left hand field pan and right click, properties.
3) Set the default value for this as show in image below....
[i set it to current user, you can pick any other field value or function etc]
