This will depend on where you are when you need to access the properties. If you’re inside a WCF service that received a message from BizTalk, I don’t believe that context data is carried over. However, with an orchestration inside BizTalk, you can access the value of promoted properties in an expression shape using the Message(PropertyName) syntax. You can also use a message assignment to set the value of the properties.
If you are interested in moving data from context into the message, you may want to look at “Property Demotion”.
Here’s an article from Stephen Thomas on how to do it:
Although the article was written for BTS 2004/2006, it still applies to BTS 2009