Got this from bcook1000 somewhere else on this forum.
Received this solution on a forum:
1. go to your settings
2. select "applications"
3. then select "manage applications"
4. press menu and select "filter" then "All"
5. scroll down to:
6. "Download Manager" then select "clear data"
Basically, you need to clear the download cache from the download manager. I had the same problem. I was able to download a few apps from the market, but after a while I kept getting the "download unsuccessful". After I saw this tip and was able to clear the cache on the download manager, I was able to download the apps again.
Good Luck
gmdphd