October 14, 2010 
In my case I tried to read a field value of type LinkFieldValue. The field type is defined in the assembly Microsoft.SharePoint.Publishing. I read the field value using this line of code:
var fieldValue = item["MyFieldName"] as LinkFieldValue;
This works pretty fine in a console test application or in a farm solution. In a sandbox solution the following error will occur:
Type ‘Microsoft.SharePoint.Publishing.Fields.LinkFieldValue’ in Assembly ‘Microsoft.SharePoint.Publishing, Version=14.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c’ is not marked as serializable.
It took me some time to solve this:
string value = item.GetFormattedValue("MyFieldName");
LinkFieldValue fieldValue = new LinkFieldValue(value);
It's not a beautiful piece of code but it works.
Thx for this hint, i’m developing an sanboxed solution for displaying all item field values… at the moment i only read them with item["MyFieldName"].toString() but with your approach the rsult could be a little bit nicer