Anyone requesting XML from the Twitter streaming API better switch to JSON as soon as possible. Starting in early December, the microblogging service will only send JSON-formatted responses from its popular new streaming API, the recommended way to get search results as they happen.
Twitter's Taylor Singletary explains the change:
We will end support for XML on all Streaming APIs on Dec 6th, 2010, after encouraging developers to use JSON-based output formats for some time now. This deprecation does not apply to the REST or Search APIs, but all clients requesting XML from stream.twitter.com, regardless of role and method
(sample, filter, follow, etc) will be affected.
As the announcement says, this currently does not affect other APIs, such as the Twitter Search API or the flagship Twitter API, which supplies user timelines and other data. However, we have to expect that this is a sign of future changes. Singletary points out that Twitter's newest APIs only use JSON, which pushes further the argument that an XML-less Twitter API is in the cards.