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First, try looking at your headers from a client to make sure that the headers aren’t being mangled or lost somewhere. You can use curl --head and then the URL to see the full headers that a client on the same computer will receive. (If you are changing behavior based on cookies or agent information, you may want to use a plug-in or the developer tools for that particular client to see the header information.)
Second, your UIDs are not unique across the universe. Those UIDs could be duplicated somewhere else. One common practice is to append your own unique hostname to the UID to ensure that they are unique.
Finally, you didn’t mention how the feed fails in Google Calendar and in iCal. Is the feed simply ignored as if it were empty? Is an error generated in either case? Or are the items showing up but not showing up correctly?