Of course I know that Pokemon games always have only one save slot.
If start a new game and try to save over your old one, you get a message that you can't. Before that, there's even a warning that you won't be able to save if you start over.
I looked in the back of the manual and there is a note on the back-up data, and it said something about needing to send it back to Nintendo if something happens to your save data. Unlike Nintendogs, there does not seem to be any kind of instructions on how to wipe the data yourself.
So, yeah, I think this is probably some anti-hacking measure of some kind, and part of an effort to make people get the Pokemon through actual trading more often? Also, I will say that this is good in that it makes it less desirable to own both games.
OK, I just tried out the GTS (Global Trade System), and it's about the coolest thing EVER. It is basically an anonymous eBay where Pokemon are the currency.
Basically, you can post Pokemon for trade on the Internet with your desired Pokemon, gender and level range and search for others.
So, for example, I wanted a Hikozaru. I picked it from a list, said I didn't care about the gender or the level. It searched for a second, and three candidates popped up - one wanted a Naetoru of any gender and level, another wanted a Pochama of any gender and level, and the last one wanted a Pochama of any gender of at least level 10.
Just for shits and giggles, I thought I'd post a totally unreasonable request of my own. I put a Geodude up, and said I wanted a Hikozaru. It uploaded my Geodude to their server, and it will wait there until someone accepts or I take it down. If someone accepts, the next time I go to the GTS it'll download to my cart.
While limited now, another cool feature is that you can set your location. Currently the crosshairs are limited to Japan, but I would imagine as the game is released in other territories those constraints would be lifted. So you see a picture of the globe, and you move a little dot around. When you place the dot, it brings up a list of cities and towns near that dot so you can select your location. Really cool shit.
Not wanting to give up either of my starters I passed, but...wow. Pretty much the only reason to have multiple carts now is to get all the legendaries. I am totally fine with not being able to restart my game to get them all now.