I'm not saying it's not for 360, but that is a terrible reason to think that it is. Nearly every high-end electronic device uses that simple for the power button. Also, why the Hell would the two exclamation points be the memory card ports? That wouldn't be very clever. They wouldn't bother doing something like this if it was that simple (nor would they use imagery that has nothing to do with the game). Also, just because it's bright green doesn't mean it's for 360 either. And the Wii one doesn't make any sense at all. I find it very unlikely that this is a platform announcement. The imagery must have something to do with the game (which is why an Ac!d Sequel seems more likely than a platform announcement). Also, the ! is pretty synonymous with Metal Gear, so even an Ac!d Sequel isn't necessarily implied either.
You guys really have to think outside the box. Why would they bother with a teaser like this if it was that obvious? Also, I would think the wording is intentional. "A" Next Metal Gear is... I'm not sure what to make of it yet, but it doesn't seem like anything anybody has said so far. We'll see soon enough.
EDIT: OMFG!!!! IT'S A METAL GEAR MUSICAL!!! IT MAKES SO MUCH SENSE!!!!!!