Worth closer inspection, today Kintera announced that they've published an open API to allow integration directly with Kintera Technology. You can view the API documentation here. Some initial thoughts:

• You need a Kintera Sphere account with the API enabled (duh).
• If you are on PHP, they recommend version 5, since PHP4 will be dead soon.
• Sample code is available for C#, Java, and PHP. Must mean that that not a lot of Kintera clients have Ruby, Python, or Perl sites.
• The API is SOAP based, which is a little heavier to program, and can be difficult to debug.
• The API has methods for describing entities (business objects), creating new records, retrieving one or more records, updating a record, and deleting a record.
• A sign that they've thought ahead, for making batch operations easy, there is a GetUpdate method to return updated records within a specified time frame.
• Seems like you'll be able to run arbitrary queries in the CRM database, and also Execute transactions.
• One potentially worrying aspect, is that the API does not apply entity access restrictions. I'm not versed in exactly what that means, but it sounds like you are connecting as the equivalent of "root" and can do almost whatever you want via the API.
• I couldn't find any mention of the API being rate limited to so-many queries or calls a day.

HT: Non-profit Tech Blog