Yes, EPiServer CMS 5 requires .Net 3 because it uses the Windows Workflow Foundation. The prerequisites is listed on this page:
http://www.episerver.com/en/EPiServer_Knowledge_Center/Documentation/Release-Notes---EPiServer-CMS-51/EPiServer-CMS---Manager-Installation-Instructions/
Prerequisites for this installation:
.NET Framework 3.0
SQL Server 2000 / 2005
Windows 2003 Server / Windows XP
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Lars Øyvind Bodahl
www.epinova.no