Congratulations from a fellow JET.
Which group will you arrive with? If you come in the August group, I'll be helping with your orientation in Tokyo. Fancy going to the hombu? You'll be jetlagged as hell though…
You've chosen a good way to experience Japan. I used to be a TESOL advisor - I advised people on Teaching English to Speakers of Other Languages and also helped them work out where to go and how to do it. In the case of people wanting to go to Japan, I always said try out for the JET programme, since it offers a pretty good deal. I've taught English in Japan and Australia before and I took my own advice last year and came back to Japan with the JET programme.
Your role could be anything from human tape-player through to main teacher. It will depend on you and your Japanese teachers of English, your level and how many schools you're at. In my experience, there is very little actual team teaching - it works out one or the other (JTE or ALT as main teacher). You'll get lots of advice when you arrive though. I hope you get plenty of opportunity to do aikido. I've got too many schools here, but a sweet location for aikido (I train at the founder's dojo in Iwama! Come along if you're nearby!).
Here's hoping you get one of the better schedules.
For now, here's a link to an online database of team-teaching lesson plans that might come in handy.