According to this thread http://world.episerver.com/Forum/Pages/Thread.aspx?id=28931&epslanguage=en it seems that EPiServer is denying responsibility to make XForms on EPiServer 4.x work with IE8 since IE8 was released after EPiServer 4.x.
The customers are experiencing an error when loading XForms with radiobuttons and/or checkboxes. The error is “invalid form” (translated from the Norwegian “ugyldig skjema”).
UPDATE 2011-01-17: Also I have experienced that in IE8, the submit button will not submit the xform which is strange, since it should be calling a javascript function: “return(false)”, and then specifying an action string.
In the comments to this posting, Björn Sållarp has proposed a solution from his blog: http://blog.sallarp.com/episerver-xforms-ie8/