I've done one major project (technically it's still in progress). I find myself using Blend and Visual Studio at the same time when working on the Silverlight project. Blend is so much better at the visual work but pretty much sucks for the code, and vice versa for Visual Studio.
I used MVVM Light Toolkit. For my next project I'll try Prism. There are other frameworks but as I understand it both of these are good for Silverlight or WPF.
I started to use MEF for some stuff, but dropped it to simplify getting something done rather than trying to get everything done. If extensibility is essential to the idea of your app then MEF seems to be a must have.
I would suggest spending some time getting good and familiar with the VisualState element, which I found simplified things a lot. Things like enabling, disabling, animating, transitioning, and so on.
The Liquid Controls (http://www.vectorlight.net/demos/silverlight_controls_demo.aspx) are nice and you may benefit from using them.
WPF might be a better choice for your Twitter app. From what I understand the typography and text support is much better in WPF than what Silverlight 4 offers at this time. If you don't plan on getting exotic with any of that then SL 4 should be fine.