The documentation for Collection<T>.Add specifically states that when calling the Add method, the new item is appended to the end of the collection. This is not necessarily true. I haven't tested it in all it's incarnations yet but it appears to be adding to the beginning (index 0) of my collection.
With Silverlight 4 and the Entity Framework you get a lot of work done on your behalf in terms of standard UI CRUD-style operations. Validations and I/O are pretty easy to accommodate out of the box. But sometimes you need to perform some long running tasks either on the client or on the server via service calls. To prevent your UI from hanging and annoying your users, you should consider placing these operations on a background thread. The BackgroundWorker object is the perfect solution for this. It is easy to use and easy to debug. There are, however, a few things you should understand about where and when the BackgroundWorker can provide feedback and/or manipulate the UI. Spending a few minutes building out the sample listed here should give you the foundation you need to implement very complex, very responsive scenarios.
I’m currently in the process of rebuilding my Silverlight application. The initial beta of this app was somewhat underwhelming in that the problems it solved were those I found most interesting, not necessarily those the beta testers were looking for resolutions to. So we head back to the drawing board, this time to build out something that benefits my users as well as provides me with exposure to real world problems that I may not encounter in my day job.