On my G1 running Android 1.6, I started to have problems with the Android Market. Any application that I tried to update or install resulted in an error message saying that the download was unsuccessful.
Luckily with a bit of googling, this is what I did to resolve the problem
1. Open Settings
2. Select Applications
3. Select Manage Applications
4. Select Market and the select Clear Cache
5. Repeat step 4 and clear the cache for Google Apps, Google Talk and Checkin Service.
6. Restart your phone
When you open the market it will ask you to login to your google account with your username and password. Once logged in , you will be able to download and update apps.
These instructions are based on:
http://www.technipages.com/android-market-download-unsuccessful-error-fix.html
