I use t-mobile my touch 3g and I keep getting 'download was unsuccessful' I found a solution that has worked twice and I will post the method here after I have tried it and it works the third time. You don't even need to reset phone.
Hurry, it has worked five out of five times so I'm going to tell youall already.
(1) Under Settings -> Applications, I have Unknown Sources checked.(don't know if that matters)
(2) Go to Settings -> Wreless Controls -> Mobile Networks, you will see Use Only 2G Network, if it is checked unchecked it, wait 5secs and check it or vice-versa, you will lose network in the process and it will be restored.
That's it, so go download, it also solves applications that hang at starting download and almost all connection issues I had with applications. It worked everytime on my t-mobile my touch 3g in Nigerian on etisalat network