Google has finally "answered" calls for a GCal API. Blog Post here and the API is here. It cover's:
- Find and Request your Calendar's Feed URL manually and using the protocol or the GData client library
- Add events using the client lib
- Send a date-range query i.e. request a set of events which match the date range you specify. A simple search.
If your developing one of these be sure to check out the Java Client Library Docs too.
Can't wait for Calendar apps.