I have been working on the issue of not being able to download Lion since the software became available through the App Store. I think I have had the "Waiting" and "An Error has occurred" messages on my system for 2 weeks straight. I was out on the web researching if there was a way to resolve the inability to download Lion. I checked several forums (some of them were even Apple Support Community Forums by the way) and came across maybe a dozen references to the Family of Modem I had in use. It was a 2Wire 2700HD-G. Much of what I read about the Modem indicated there was a known issue with the Modem not being able to download files over 2Gigs without losing the connection. Mostly related to xBox 360 owners not being able to download large video games or others not being able to Download HD Video files over 2Gigs. I contacted my Service provider from whom I purchased the 2Wire Modem to see if they could offer any information about this issue.
As you might expect they would not or could not confirm the problem. I chose to lease a new modem so I could at least test a different brand (Actiontec PK500) and it arrived today. I tried one more time to download Lion with the 2Wire Modem and predictably it failed.
I connected and configured the new non 2-Wire Modem. Clicked resume on the App Store Menu and the installer immediately began downloading. Currently I am a few minutes from completing the Lion installer download.
While this was not necessarily scientific problem solving, it clearly resolved my my installer issue.
I hope this helps!