I have had the same problem for four weeks. I wrote to Apple, even called them in the US (I live in Germany) und today I got really mad and used a word in my email (BS) which Apple didn't like.
An hour later I got a responde from them telling me, that the reason I am getting this message:
Your Purchase Could Not Be Completed.
For assistance, contact iTunes Support at www.apple.com/support/itunes/ww/.
is, that I do not live in the USA and therefore am not allowed to make anymore purchases. It's a joke, since I have been buying at the US iTunes Store for years, but if Apple doesn't want my money anymore, that's OK.
So if the people having this problem do not live in the USA, it makes no sence for you to contact the iTunes Store. I have been doing this for the past four weeks (a total of 10 times) and after three days of being able to purchase again, I got the same message again and again.
The following link was added to the email I got earlier from Apple:
http://www.apple.com/legal/internet-services/itunes/us/terms.html#SERVICE
and I quote out of it:
REQUIREMENTS FOR USE OF THE ITUNES SERVICE
This iTunes Service is only available for individuals aged 13 years or older, unless you are under 13 years old and your Apple ID was provided to you as a result of a request by an approved educational institution. If you are 13 or older but under the age of 18, you should review this Agreement with your parent or guardian to make sure that you and your parent or guardian understand it.
The iTunes Service is available to you only in the United States, its territories, and possessions. You agree not to use or attempt to use the iTunes Service from outside these locations. Apple may use technologies to verify your compliance.
So I guess, unless Apple changes it's policy, Millions of users living abroad won't be able to purchase anymore in the US iTunes Store. Maybe a Proxy will be the solution?