Yes, although not all of the activities are optimized for offline use quite yet.
Your MOOD and HEALTH ratings will be recorded and uploaded to the server the next time your are connected to the internet. The activities in RELAX will also work correctly, as should GOALS.
THOUGHTS will work but it will not persist the audio files since there is no connection. You may use it to analyze your thoughts at any given time however.
Your MOOD HISTORY is only available when you are connected to the internet unfortunately.