I am the intranet developer for the company I work for and I have been doing this for the last 5 years. My projects are divided into two solutions, the "Intranet" solution itself and the "Library" solution. The "Library" sln itself has several projects containing the DAL, BLL, etc.. The reason why I kept them in a different solution is because I thought that "maybe", one day my library sln can be used in other projects as well - you know reuse the code that I already wrote :) Well, that never happened. Now, since its so easier to have all projects in the same .sln, I am thinking to just do that. Is that a wise situation? What would you do if you were in my shoes?
In the past I've used and reused the same 'project' in multiple solutions - I really just see a solution as a 'particular' instance of a collection of projects.
For example, we might have different solutions for the same overall piece of software depending on whether we want to be doing unit testing (in their own project) and or integration testing (in a separate project), and we'd open the right solution for what it is we're about to do. That way if you're doing normal coding with unit testing you don't have to build the integration test code every time and visa-versa.
Only thing to watch out for is bringing in a project to a solution that is a dependency of lots of other projects/solutions and then "accidentally" changing the code in it without realising it's in a side project rather than your main code. Then you can start breaking loads of other projects that depend on it and not realise!