I was working with a ORM system the other day and kept running into problems with the Nullable types when I called Convert.ChangeType().

[InvalidCastException: Invalid cast from 'System.DateTime' to ' System.Nullable`1[[System.DateTime, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]]'

A Quick Google search came up with a number of entries.  This is a known problem in the CLR.

Here a couple or other blogs on this and below is my version of the code:

Peter Johnson's Blog
The PumaCode.org Blog

        public static object ChangeType(object value, Type conversionType)
        {
            if (conversionType.IsGenericType && conversionType.GetGenericTypeDefinition().Equals(typeof(Nullable<>)))
            {
                if (value == null)
                    return null;

                NullableConverter nullableConverter = new NullableConverter(conversionType);
                conversionType = nullableConverter.UnderlyingType;
            }

            if (conversionType.IsEnum)
                return Enum.ToObject(conversionType, value);
            else
                return Convert.ChangeType(value, conversionType);
        }