Often when developing solutions, I endup with a solution that wants to be deployed globally. Often this is not wanted, as a solution mostly should be available for only one webapplication.
Turns out that a solution will be deployed globally by default. When the solutions contains a safecontrol registration, then the solutions will be able to be deployed per webapplication.

Koen has a nice post about this: K&Y – How to prevent SharePoint 2010 solutions from globally deploying.