I'm somewhat torn on this issue, but my gut feeling tells me that the plan Chris is describing is the right one - to eventually make them ultra rares, but to never again give them out for free in a co-op or some similar manner.
I have two strong beliefs which are in a bit of conflict because the designers of this game did such a damn good job of meshing different styles of games into one.
1) I completely agree with trying to give some competitive balance to players who play for free. I do not think players who spend money should have exclusive access to super sick cards (or items in an mmorpg). I am completely in favor of the premiums eventually becoming ultra-rares. Besides, some ultra-rares in the current card selection sell for many points. I would assume premiums which become ultra-rares would still sell for many points if the card is good enough to warrant it.
2) One of the coolest feature of collectible card games is when they mature and have had multiple expansion packs, early powerful cards become less common, more valuable, and even can take on a somewhat mysterious aura. Not sure what the long term plan is for the cards we get in packs now... if the packs will cost a lot more (this would get my vote) or if they would still be available at the same price or if they would simply disappear...?
These two competing beliefs can work together. Simply consider Premium cards as ultra-rares which we got early, and if the format allows for all ultra-rares to go up in value over time, then both premium cards and ultra-rares will go up in value over time, and there is no problem.