First here are the two Bruces that have popped up so far:
first one:
Now here's a Jay Bruce highlights card I pulled from a pack today:

Until I see a photo or a video of one of these buggers, I'm calling it a hoax. I'm not saying Topps wouldn't do a gimmick like this as they quite clearly have in the past, but the fact that the foil doesn't look right combined with the sheer goofiness of the error (seriously, having a reverse negative slip by is a common mistake, but inverted?? that only happens with stamps) makes me think Topps is in the clear on this one. I will likely be proven wrong (Hell, they might be printing a mess of 'em now to cash in!) but I say these aren't real.
UPDATE: Two more of 'em, same image, same gradation pattern, RU is darker than the rest of the letters, bunk bunk bunk.
one:

I don't understand how all four cards could have that same 'shine' pattern unless it's a hoax.
Possible non-hoax reasons for the sake of argument:
- By sheer coincidence the first guy to pull one is the only seller with a scanner or camera of any kind and the rest just used his image.
- There is no foil on the card and the name and logo are printed on the card that way in ink.
- I have turned into a conspiricy theorist lunatic.
Until I see an image of the card that doesn't look exactly like all the rest I say bunk. Bunk! BUUUUNK!
Another Update:
Two Kosukes, both with the same gradation (darkest on the Ks), one scanned, one appears to be a photo. One, two.
Now, if the card have no foil and the name is printed on with silver in, then yeah, the cards are legit. Topps has inserted cards into the base set at the last minute without foil before, I seem to remember a Boston Red Sox World Series victory base card from a few years ago that had no foil. HOWEVER - if that's the case than these are NOT error cards and are instead bullshit gimmick cards. Why would a corrected error originally be printed with ink instead of foil when the rest of the base set has foil for the names? Basically what I'm saying is:
The name and the logo on the inverted cards do not appear to be in foil on any of these auctions.
If the cards were not printed with foil stamping, that means they are either fakes or Topps had no time to add the foil to the cards so they used ink instead.
The more cards that show up on the market (I've seen 4 so far) makes the likelyhood of a fake inverted card conspiracy much more unikely.
If they are real and have no foil, then that means Topps inserted them as a gimmick and they are not in any way an error.
In other news there is a variation in the David Cone Ring of Honor card as well:
David Cone
David Con
This I think is a legit error. Or at least plausible enough to be a legit error.
Yet another Update: It just keeps getting weirder...
Note again the foil. The Topps logo looks like all the others. Topps got cute and inserted these at the last minute. You'd think they could at least do some long-term gimmick planning up there. Another hat tip to Holy Hitter, who's been tracking these auctions down all day.