3X3 grid, as displayed above.
The idea is that it'd look just like a dirt block, but if you step on 'em, they explode.
Now I know what you're thinking. "Josef, you sleek, well-oiled muscleman!", (thank you)," why would TNT go off when you step on it? That's Silly with a capital s, as displayed here!"
Consider this. Currently, you set off TNT by giving it a good whack.
So, why wouldn't it go off if you step on it? Maybe TNT as it's used in Minecraft is an impact-sensitive substance.
So many people would set random ones everywhere and people would start getting pissed off because they keeps losing all their items to Fake TNT. But I like the idea XD
Well, they still take time to detonate, of course...so you could run away and minimize the damage (if you're quick enough, you tub of lard!). But I think the first time you step on an inccouous little brick of dirt and hear "*pchk* SSSSSSS", your first instinct would be to look around for a creeper. Little does the noob suspect the explosion comes from below! *evil laugh*