I know you've seen my question about the android restful API service, and think you can do something similar the the answers stated there. You'll need to use a Handler to execute a timed callback from the service.
Also, since that question was answered I've discovered google buffers which may be worth you looking at:
Protocol buffers are a flexible, efficient, automated mechanism for serializing structured data – think XML, but smaller, faster, and simpler. You define how you want your data to be structured once, then you can use special generated source code to easily write and read your structured data to and from a variety of data streams and using a variety of languages. You can even update your data structure without breaking deployed programs that are compiled against the "old" format.
There are some good examples on the tutorial pages - it'll make your chat client super fast!