If your like me, then you may have a little OCD and need everything to be completely organized in your environment. BIDS has been annoying me in this respect lately. I am working on creating a data warehouse and will be starting on the next portion of the project shortly. I wanted to keep everything for it in the same solution, my warehouse ETL as well as the work on the Cube. So I figure, create the solution and add a new project for the Cube. So I create a new project and much to my dismay the solution explorer replaces my current project with the Analysis Services project. Here is the way to get around that and show all your projects for a solution at the same time. Click Tools and select Options. Choose Projects and Solutions on the left panel. Check the box next to Always show solution. 
Now when you create a new project inside the solution, it will not remove the old project, it will show them together! Enjoy!