As Leah Culver reports on the Pownce Blog a first version of an API has been released now. Unfortunately mainly for reading notes but not for posting. This should come soon. It might be interesting to see how they handle event posting and file uploads as these are the things which sets Pownce apart from Twitter.
If you want to participate in this you can join the Google Group around it (although not too much discussion seems to go on there but maybe you are the one to change that).
Now there is a more inofficial Python API already which basically reverse engineered the way the hidden API for the AIR-client worked. So I guess it’s only matter of hours before a more official one comes out ;-)