Make possible accessing the caret position inside a SpanLayoutBox.

Resolution: 
This issue is addressed in the new version of the control - RadRichTextEditor. Please use the new control instead the RadRichTextBox. 

You can use the following code snippet: 
DocumentPosition caretPosition = this.radRichTextEditor1.Document.CaretPosition;
SpanLayoutBox spanLayoutBox = caretPosition.GetCurrentSpanBox();
int pos = caretPosition.GetCurrentPositionInSpan();