Right, something strange is going on!
I have now set Allow HTML in Product Fields? to No.
Here is a product that I have added the code for today (after upgrading to 3.1 last week):
http://www.photographyblog.com/revie...&cat=29&page=1
Here is a product that I have added the code for before upgrading to 3.1:
http://www.photographyblog.com/revie...&cat=29&page=1
As you can see, in the first example the code is correctly shown as plain text.
In the second example, the code is "incorrectly" shown as working HTML. If I choose Edit Product, only <br clear="all"> is shown. This also applies to all of the other products that I edited in this way prior to upgrading to 3.1.
It's as if all the old, pre-3.1 products are doing the opposite of what they should do!