This post assumes basic familiarity with Java and the Android SDK.  It gives you a brief intro on how to talk to the NFC API to read a tag and subscribe to an event that detects it.  

Setting up the Activity

[java]
public class Main extends Activity
{
  @Override
  protected void onCreate(Bundle savedInstanceState) {
     super.onCreate(savedInstanceState);
     setContentView(R.layout.read_tag);
  }

  @Override
  protected void onResume() {
     //Process the intent that started this activity. It should be a Tag Discovery related intent
     this.parseIntent(this.getIntent());
   }
[/java]

Processing the NFC Tag event

[java]
boolean parseIntent(Intent intent) {
 String action = intent.getAction();

 if (NfcAdapter.ACTION_TAG_DISCOVERED.equals(action)) {
    Tag tag = (Tag) intent.getParcelableExtra(NfcAdapter.EXTRA_TAG);
    boolean success = FlomioNdefHelper.writeUrlToTag(url, tag);
 } else {
    return false;
 }
}
[/java]

AndroidManifest.xml

[xml]
 <uses-permission android:name="android.permission.NFC" />
[/xml]

[xml]
<activity android:name=".ReadNfcTag" >
 <intent-filter>
  <action android:name="android.nfc.action.NDEF_DISCOVERED" />
  <category android:name="android.intent.category.DEFAULT" />
  <data
   android:host="flomio.com"
   android:scheme="http" /></pre>
 </intent-filter>
</activity>
[/xml]