Receive XML post to Salesforce Sites page
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I'm currently looking at building a lightweight integration between PivotalTracker and Salesforce.com. Reviewing [this bit of PT documentation](http://www.pivotaltracker.com/help/integrations#activity_web_hook), it looks like I can do an update of Salesforce data based on PT activity. Awesome! I can't figure out how to access the XML data that is being posted however.

I can see anything in `ApexPages.CurrentPage()` that looks like it will let me get to the XML. Has anyone done anything like this, without the use of an intermediate server?