I have found an article about using Usercontrols in SharePoint 2007. The writer describes a way to encapsulate a usercontrol in a webpart. This way, a developer can have a visual preview of how his web part will look like.
And a link for creating SharePoint 2007 webparts in VS2005:
Mart Muller’s Sharepoint Weblog – Create a SharePoint 2007 webpart step by step
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