The subject line pretty much says it all, but the pictures of Power Girl on this web page show you what I mean. (They’re pretty much safe for work as long as you aren’t employed by Jerry Falwell.)
Is there a specific name for this fashion choice?
It’s called a “keyhole” although the ensemble in that particular pic is for a very large key, apparently.
Here’s a pic, sans comic book characters:
http://thehousewife.blogsplot.net/wp-content/uploads/2006/08/neck_heyhole.jpg
I’ve never seen one that extreme in a real garment, but allowing for the comics’ exaggeration convention, I’d probably call it a modified “keyhole”. A classic keyhole collar has two panels which may button or zip up top and the cleavage revealing hole on the bottom.
Keyholes can be anywhere from artsy-fartsy to sweet girl next-door to insert dollar bills here to recieve blowjob, please.
Thank you for posting these.
Many thanks for the answers. I’m trying to describe a far more modest than the Power Girl one, but not knowing the appropriate name it was hard to google for it.
Nitpick: Your second link (the “sweet girl-next-door” version") is technically not so much a “keyhole” neckline as a “notched” neckline, because the two ends of the keyhole aren’t touching. A true keyhole style shows a closed simple curve, topologically speaking, juxtaposed against the uninterrupted curve of the neckline; a notch, on the other hand, cuts away part of the neckline itself. A more extreme version of the notched neckline is shown here.
And if you ask me, PowerGirl in the OP’s picture looks like a blonde version of Robin with a parasitic conjoined twin’s ass growing out of his chest. Weird.
I disagree, although I did consider the notch/keyhole debate before posting. I finally decided that the definitive characteristic of a keyhole neckline, in any pattern book I’ve seen, is that the seamstress must cut a curve out of the front of the pattern piece, not just cut in a straight line down before hemming. The t-shirt I linked to shows a definite, albeit small, curve to the “hole”; it’s not a straight notch. If you brought the two top points together, there would still be a gap - you couldn’t put the whole edge together without stretching or distorting the fabric. I have several identical keyhole T’s which have a tiny button up top to close the points if desired, but generally they’re worn open.
I didn’t know that one of Supergirl’s powers was the ability to have wind swirling her skirt upward even when she’s standing still inside!
It would make Heroes a much more interesting show when they rip off that superpower. 
No one asked you, but I agree the pics are a bit grotesque, golly-I-never-dated-in-high-schoolish. 
WhyNot: Yeah, I see what you mean, but I still maintain that a keyhole is a cutout whose endpoints touch:
A curved/oval notch may be called a “keyhole notch” if you like, as in this description, but it is not an actual keyhole unless the endpoints touch. That’s my couture-nazi take on the subject, anyway.
This has always been my understanding as well, with a “notch” being something totally different.
Alright, I think I can compromise on “keyhole notch”. Care to broker peace in the Middle East next? 
(Mostly I was trying to find a “keyhole” that wasn’t slutty, to show the range of possibilities.)
Now I’m just wondering what sort of key would fit the keyhole. I’m not sure I’ve ever seen a ta-ta key…or maybe I did but just didn’t know what it was.
Yup, but I think a keyhole top is pretty much supposed to be slutty. To show the range of possibilities, as it were.
Just look in any Tata key cabinet.
That’s great! You truly can learn everything on the Dope.
Ummm… how many dollar bills?
I don’t know what the cutout is called, but the space between closely-pressed breasts is called the Inter-Boobical Groove.
Not to be confused with the Inter-Biblical Groove, which is the space between the Old and New Testaments.
AKA “cleavage”.
A “keyhole”, eh? I just figured it was the entrance to a parallel universe.
Taco Bell.