Multiupdate is a very powerful feature and therefore it should be handled with care. For a more safe way to work with it, it would a great benefit if there could be a mark when there are diffrent values in the field.
Example:
There are a couple of records to be updated with three fields: color, value, size.
All selected records have the color red. In multiupdate, the field color contains the value red.
The field amount is empty (NULL) in all records. Multiupdate displays this field as an empty one.
Now there is another field called size and the records have many different values. Unfortunatly, multiupdate display this field even empty, like the one above, but it contains different values.
There is a big risc that someone isn't carefull and overwrites the values by accidant. To avoid this, it would help if fields that contain different values are marked with a little exclamation-sign or something else.
In addition, perhaps it is possibly to create a tooltip that shows a hint with something like: "There are 5 diffrent values"