- Using Son of SmartPart will allow you to run ASP.NET 2.0 UserControls and WebParts in SharePoint 2003.
- SharePoint 2003 web parts will not work in ASP.NET 2.0 pages with Web Part Zones.
- SharePoint 2003 web parts will work in SharePoint 2007.
In addition, take time to review Dino Esposito's article "WebParts: From Sharepoint to ASP.NET 2.0". It discusses the similarities and differences between the two models.