I hope this hasn't been covered before -- I searched but didn't find this exact scenario.
I have a "play" phone that doesn't have service that I keep in airplane mode most of the time because I just use it as a computer (not a phone) and use wifi for Internet.
I had a problem today where the phone wouldn't install anything from the marketplace and would try and just give "download unsuccessful".
If the phone stays in airplane mode too long the time gets pretty off, like by hours, and I noticed that was the case. So I turned airplane mode off, let the time sync up, and then turned airplane mode back on. Even without having service assigned to it the phone picks up the time from the network apparently.
Anyway, after that I was able to install apps from the market.
I don't know for sure that it's the time being out of sync that mattered, but maybe if the time is way out on the phone some sort of something just isn't happy.
Anyway, just a note for anybody having persistent "download unsuccessful" via WiFi. See if your time is off and if it is, get it in sync and try again.
Not sure if this will help anybody or not. Maybe an odd scenario that most people would never run into.