You're right about the Subscribe widget not validating because it's within a list without actually being a list item itself. Thanks for reporting.
If all widget-ready themes use lists, this should be really easy to fix without messing up the validation of themes that don't use lists. If list items are not a standard element (or wrapper) for widgets, we're going to have to get crafty to make sure all valid themes continue to validate... just to give you some insight. :)
Most likely you'll see a fix for your theme (and the standard WordPress theme) in the next release. Thanks again!