Figurines are console generic, and may be used with multiple consoles. The figure actually holds data (your stats) from the game, and may be carried to your friends house and played with on his console, even if he has a 360 and you have a Wii. So yes, your figure will work on both, and your figure will keep persistent data and stats regardless of which platform you use.
The Portals of Power have a hardware dongle for hooking up to the consoles, and the games/discs are each written for each console exclusively, but that is where the console exclusivity ends. Wombat says the hardware interface technology may be the same one used with Guitar Hero, and knowing Activision that is likely the case. Note, the 3DS Portal is physically smaller than the others.
Toys For Bob is the original designer and developer of this game. When Activision realized that this game was something special, they decided to go multi-console and enlisted other development teams to create console specific versions. I don't know if they are "ports" per se, or if they were built from the ground up for each platform.
When it comes to store exclusives, so far Best Buy is the only one offering an exclusive "arena." The packaging does not note it, but the store signage does. Everyone else has exclusive figurines. This is likely the one I will try and score for my kid, and we'll pick up the figures as we find them.
The game is a 3D hack and slash, like a kiddie version of God of War, with lots of variety in the game levels/spaces. Each figure has their own special abilities and attacks. If you watch the videos, it will quickly become apparent what kind of gameplay it has.
Where the game is a little malicious, is that it is full of locked areas, and these are accessed by the figures and accessories you buy. As you play the game, you will encounter locked areas that you cannot get into, unless you have the right toy. I have not yet heard if all content is on the discs, or if any of it is downloadable--I would speculate everything is on the disc as this would reduce development costs.
As the figures are all $7.99, I think they will sell well. Kids and parents are used to spending a lot on figures (thanks Kenner and Star Wars) and I think it will be an easy sell. The $8.00 price point also makes it an easy toy to buy your kid just to make them shut-up while you shop.
This will likely be one of the biggest toys this season. If Activision has any brains, they will get this toy featured on Ellen, Jimmy Fallon, or something like that. The right marketing can make this huge.