I recently started getting Download unsuccessful errors on my Motorola Droid whenever I tried to upgrade an app. Luckily, the fix was easy.
1. Open Settings.
2. Select Applications.
3. Select Manage Applications.
4. Select Market (You may have to press Menu > Filter > All to see it)
5. Select Clear cache.
6. Back out of the screen and select Download Manager.
7. Select Clear data.
8. Also clear data for Google Apps, Google Talk, and Checkin Service.
See if downloading from the Android Market works now.
Of course, this works with any Android phone (Evo, Droid X, Samsung Moment, etc.)
Original post: http://www.technipages.com/android-market-download-unsuccessful-error-fix.html