Here are the release notes from the 2/20/2012 Constant Contact API release.
Response code should be 404 instead of 400 while trying to retrieve Event details for a non existent Event
Previously an attempt to GET an Event that does not exist would return a 400 error. This is incorrect - the correct error code is 404, which is now being returned.
Blogs - Cannot Save Campaign when Inserting Blogs into a block when setting Yes to "Do you want to delete block contents before inserting?"
When using the RSS editor block, if the user responded Yes to the question on deleting existing block content, it was impossible to save the campaign. This has been corrected and campaigns can now be saved.
Entering a space " " in statecode OR countrycode allows bad data in to the database causing 400 errors
Space characters in statecode or countrycode were causing 400 errors when adding contacts. This was corrected to trim space before adding contacts.