I should mention that I attempted to post a Revision that included a list of Assignables (specifying only the Id), but the Revision still doesn't show up on the UserStory. E.g. for UserStory #29659, which is part of Project #3481, I post this Revison
<Revision>
<SourceControlId>c55088c534d0</SourceControlId>
<CommitDate>2013-05-21 10:33:21 -0400</CommitDate>
<Project Id="3481" Name="My Project"/>
<Author Id="11">
<FirstName>Scott</FirstName>
<LastName>Palmer</LastName>
</Author>
<Assignables>
<Assignable Id="29659"/>
</Assignables>
</Revision>
The post succeeds, but the Source tab for UserStory #29659 is still empty.
Regards,
Scott