If you receive a "Download unsuccessful" message when attempting to download an app, try the following troubleshooting steps to resolve the issue:

• Restart your phone then try downloading the app again.
• Try cleaning Market cache like this:
  1. Settings -> Applications -> Manage Applications
  2. Open settings for Market app
  3. Tap [Force Stop]
  4. Tap [Clear data]

If you're still unsuccessful, let Google know. Some users have had to Factory Reset their phone to retrieve app updates as a result of the recent Android Market update.