Bill de hÓra has put up an excellent post:
Speculation: for Data APIs in 2009 there will be two developments and one debate.
The two developments (or rather ideas) Bill talks about are putting links into API data and standardisation of feed metadata; he predicts the debate will be around the use of custom data formats.
The data format issue has been debated quite a bit in the REST community recently. My guess is that we'll see more and more standardization of individual fragments than complete formats. Bill mentions RFC 5005, which is an excellent example of this: although it refers to Atom, the elements it defines can be used within any XML format.