How to best save InApp purchase status locally?

I too was making the same investigations, but during my testing I figured out that you do not need to store it, as Google do all the caching you need and I suspect (though I have not investigated it) that they are doing so as securely as possible (seeing as it in their interest too!)

So here is what i do

// Done in onCreate
mHelper = new IabHelper(this, getPublicKey());

mHelper.startSetup(new IabHelper.OnIabSetupFinishedListener() {
public void onIabSetupFinished(IabResult result) {
      if (!result.isSuccess()) {
         // Oh noes, there was a problem.
         Log("Problem setting up In-app Billing: " + result);
      } else {
         Log("onIabSetupFinished " + result.getResponse());
         mHelper.queryInventoryAsync(mGotInventoryListener);
     }
    }
});

// Called by button press
private void buyProUpgrade() {
    mHelper.launchPurchaseFlow(this, "android.test.purchased", 10001,   
           mPurchaseFinishedListener, ((TelephonyManager)this.getSystemService(Context.TELEPHONY_SERVICE)).getDeviceId());
}

// Get purchase response
private IabHelper.OnIabPurchaseFinishedListener mPurchaseFinishedListener = new IabHelper.OnIabPurchaseFinishedListener() {
    public void onIabPurchaseFinished(IabResult result, Purchase purchase) 
    {
       if (result.isFailure()) {
          Log("Error purchasing: " + result);
          return;
       }      
       else if (purchase.getSku().equals("android.test.purchased")) {
      Log("onIabPurchaseFinished GOT A RESPONSE.");
              mHelper.queryInventoryAsync(mGotInventoryListener);
      }
    }
};

// Get already purchased response
private IabHelper.QueryInventoryFinishedListener mGotInventoryListener = new IabHelper.QueryInventoryFinishedListener() {
    public void onQueryInventoryFinished(IabResult result,
       Inventory inventory) {

       if (result.isFailure()) {
         // handle error here
           Log("Error checking inventory: " + result); 
       }
       else {
         // does the user have the premium upgrade?        
         mIsPremium = inventory.hasPurchase("android.test.purchased");        
         setTheme();

         Log("onQueryInventoryFinished GOT A RESPONSE (" + mIsPremium + ").");
       }
    }
};

So what happens here?

The IAB is set up and calls startSetup, on a successful completion (as long as it has been run once with an internet connection and is set up correctly it will always succeed) we call queryInventoryAsync to find out what is already purchased (again if this has been called while online it always works while offline).

So if a purchase is completed successfully (as can only be done while online) we call queryInventoryAsync to ensure that it has been called while online.

Now there is no need to store anything to do with purchases and makes your app a lot less hackable.

I have tested this many ways, flight mode, turning the devices off an on again and the only thing that messes it up is clearing data in some of the Google apps on the phone (Not likely to happen!).

Please contribute to this if you have different experiences, my app is still in early testing stage.