Just to keep everyone in the loop, after a week of testing, preliminary results with SmithCart v4.26 and DotNetNuke 6.0 CTP2 look very promising. DotNetNuke 5.6.2 was the last vb.net/5.x version and all efforts are now on 6.0 and c#.
DNN 6.0 Major Highlights
Core ported from VB to C#
Popup Edit Windows
Page Admin (via drag/drop)
Form Patterns
Module Branding
Telerik ASP.net AJAX
DDRMenu
The DNN 6 beta release is expected around the end of April.
FWIW, I have been working with DotNetNuke for 8 years (since v1) and using SmithCart since Q3 2010.