In the Gmail interface, if you are in the middle of a "conversation" there is a black arrow to the message you are currently working with. That way you can put the arrow on a specific email and then hit enter to expand that message. The current Sparrow interface doesn't have the black arrow to see where you are in the conversation.
That being said, if you are in the middle of a thread (where there are new messages appended to an already thick stack of messages in a conversation that is collapsed when you reopen it), the black arrow should be by default placed on the new (non collapsed message), as Gmail does.
This is all for consistency in between the two platforms, and the need for the keyboard shortcuts (the best feature of Sparrow IMO) to function exactly the same as Gmail's.