I saw this funny little behavior today and thought I would share. You may be building applications for SharePoint 2010 with Visual Studio 2010 now and pretty happy with the solution packages (.wsp files) it creates for you. This is definitely one of the great new features in Visual Studio 2010. I was working on something today though and noticed that even though I kept rebuilding the solution (and even after cleaning it) the WSP file was not getting rebuilt. I am testing it on another farm so I was trying to rebuild and push it over to that farm for testing. It turns out that just rebuilding your project or solution will not rebuild the WSP file too. The one way I've found to do that (and I've only tried one way - there may be others) is to choose to Deploy the solution again from Visual Studio. THAT'S what causes the WSP file to be rebuilt. Just thought you may find that handy someday.
Should be a 'package' option under build/deploy on the project context menu. Been bit by that too ;)