As you have probably noticed, the Log for Life iPhone app in the App Store can't save new log entries when running on iOS4. We have updated the app to handle those issues and submitted an update to Apple, so once the changes are reviewed and released by Apple the fixes will be available.
We're sorry for the time that the app has been unavailable for use on iOS4. Until the update is available in the App Store logging by email is a great workaround.
Update, 7/19/2010 - The update is up in the App Store.