If you are doing WSS or MOSS development, do check out the VSeWSS 1.1 release. It provides some help to develop .WSP package right inside visual studio. The new 1.1 release allows you to have more control over the WSP solution creation.
If you have been only using Sharepoint designer for customising WSS or MOSS or have your own scripts to generate and maintain sharepoint features, give VSeWSS 1.1 a go. See the sharepoint team blog entry at http://blogs.msdn.com/sharepoint/archive/2008/02/11/announcing-the-final-release-of-vsewss-1-1-and-the-upcoming-version-1-2.aspx