Joshua Block a senior staff engineer at Sun and author of Effective Java gave a great session on API design.  Joshua highlighted that good API can be great company assets because good APIs capture and retain customers.  The talk was particularly enjoyable because Joshua illustrated his points with reference to good and bad Java APIs.  You can read the full session write up by James Robertson.

I love the challenge of API design.  As Joshua mentioned, it's incredibly hard, and when working on large public projects like .NET or Java you only really get one shot at it.  Once it's out there it's extremely difficult to make changes to it.

This is where I think the Indigo team is on a great thing.  Steve Swartz describes his role as making sure that the Indigo programming model is easy to use.  Don Box mentions today that he sees his role as removing the need to have to absorb complex detail in order to create working programs.  This team knows how important it is to get the API correct.  As Don said in January "the APIs are the real lock in".

I was surprised that Joshua didn't say more about the need to test the APIs with a developer audience.  He did mention that when designing and interface for others to provide implementations for, it's worth using the 'rule of threes' and writing three implementations yourself before shipping the API to ensure that it's possible.  But he didn't really say anything about API usability testing, the use of personas or the need to get APIs out into the community for feedback.  These are all things that I believe Microsoft is doing incredibly well with Longhorn (I'm not really in a position to say regarding Java, but would be interested to know what goes on there).