A while ago (August 2007) we announced that the new Data API:

http://apiblog.youtube.com/2007/08/new-youtube-api-released-into-wild...

At that time we said:

"The old REST/XML-RPC API will continue to work for at least a year -
through
August 30th, 2008. After that, its existence is not guaranteed"

Since we are now 8 months past the "deadline" (and 1 year and 8 months
since we deprecated the API) we think it is finally time to start to
put the Legacy API to rest. (Pun only partially intended.)

Over the next few days we will start to wind down the Legacy API,
which means that anyone who hasn't migrated runs the risk of having
their application break or stop working. I understand this might be
upsetting, but unfortunately it is necessary that we sunset the Legacy
API so we can better use our resources to provide you with a better
developer experience using the Data API.

Cheers,
-Jeff