Responding to a question from Bernd Wechner about some performance issues he was having while accessing the work item store, Jeff McKune posted some great information about how the work item Object model fetches work item data from the server. If you do a lot of work item access in your code and wish to optimize it, there are a lot of goodies on this thread.
Jeff talks about what data the OM fetches from the server and when more round trips are performed (if necessary) among other things.
You can read the full thread here