Sounds mad, but even though we have CS3 and most other graphics/design packages, we use the drawing tools in Powerpoint a lot to do quick web page layouts or site designs, regardless of what PP's actually meant for.

The reasons are this: anyone can use the tools without have to learn anything, it's really quick, the way it works tends to make you keep things simple (very important), you can create a template page and then cmd+V it across all the other pages (slides), you can scroll through the pages (try doing that in Photoshop), easy output to pdf/jpgs etc if you want to share and of course if you want to present the design...well 'nuff said.

If you're designing for Twitter I'm assuming that graphically you're doing something very clean and simple. This method helps you keep it that way.