*shows you the playtest version of this power*
The playtest version of this power DID have the weapon proficiency bonus in the attack line, suggesting that you always got the bonus (not just when you are proficient with the weapon). Since that changed, Flip's approach seems to be the right one. I wish WotC would have commented specifically on the changes between playtest and release, though.
You said, in your opening post, the power has the weapon keyword.
So I answered based on that.
As such, I looked at the power, because subsequant talking had suggested it had not the weapon keyword.
Now, I see that it STILL has the weapon keyword.
And as such, the attacks of the Dancing Weapon -are- attacks from a power with the weapon keyword, and as such receive the proficiency bonus. Said bonus is not a temporary bonus, so the Summoning keyword does not take that away.
Unless you want to make the point that weapons only -temporarily- have the proficiency bonus? To which I say 'Name one weapon where it's changed, ever.'
EDIT: Of course, not being proficient in the weapon denies you that bonus.
EDIT2: And I've seen the playtest. Things get changed because of reasons as the above... the power had Weapon AND Implement, but as Artificers became more weapon-based in one of their builds they scrapped the dual keyword thing... also because they didn't want confusion on whether they could use two implements for the same power.