Will CSS 3 still allow omitting final semicolons?

I think that it's just a mistake when writing the specifications, and that the real intention was to have it as a separator, just as in previous versions. I expect this to be adjusted in later drafts.

The W3C validation service says that the example without the trailing semicolon is valid CSS level 3.

I will however, as I have always done, end each declaration with a semicolon. This is a good practice, so that you don't forget the separator when you add more declarations.

Looks like there currently is an ambiguity in the spec.

You correctly pointed out that 2. Syntax Description section prescribes ending every rule with a semicolon:

Each declaration has a property name, followed by a colon and the property value, and finished with a semicolon.

And at the same time, description of the parsing automaton in section 3.6.8. Declaration-value mode reads that a closing brace without a semicolon correctly ends a declaration and current rule at the same time:

} token

Append the current declaration to the value of the current rule. Pop the current rule from the stack of open rules, and append it to the value of the new current rule. Switch to the current rule's content mode.

So according to 3.6.8 trailing semicolon is optional.

I can't say about actual intention of the specification authors. But current situation should probably be reported and fixed. Most certainly they don't have intention of breaking the compatibility with CSS 2.1 and will reword their Syntax Description section in the final version.