In response to questions:
Probably it will be in AS2. Even though I've got CS4, making it AS3 would restrict it to the nerdier coders as well as those with CS3 or CS4, which wouldn't really be fair for people with Flash 8. No big deal though, right? The AS3 coders all should know AS2 as well. :D
The communal .fla will be in Flash 8 format, but as I said I also have CS4, so feel free to save it in any format.
There are only a couple of variables you'll be allowed to change - the X and Y of your robots, their rotation, and the ball - if you're right in front of it. You won't be able to shoot missiles or crap at your opponents, as that's just unsporting. You're welcome to access, but not modify, any other variables. This would allow for a great deal of openness for the competition, but keeps everyone on the same level by not allowing "super ultra mega bots" - this is a game of scripting skill.
It's nice to see some people are interested - I'd better get working on the engine :D