I want to get another HD camera to mate with my HD100 (keep those snickers to yourself, perverts!). Any suggestions? Would a 1080i camera scale to 720p well enough to match the HD100? Or should I stick with something like the HVX200 which can shoot 720p?

I always think the first thing is to decide the precise uses you will put the camera to and what exactly end product(s) you will need.

My first thought for a second camera would be an HD200 with the wideangle lens, or the HD250.

If I wanted something much smaller with autofocus and stabilization -- such as to run around with and get quick handheld shots, anchor to the hood of a car or motorcycle, attach to a clamp and put on the apron of a stage -- I'd look at the small cameras.

If I were doing static shots, such as interviews, and I knew the workflow to match the footage, I might choose a Z1U, so I would have the 1080i when I wanted it, so I would have options for shooting something that would go to broadcast, etc.

One consideration has got to be compatible decks. The JVC deck won't play 1080i, but I think the Sony decks will play 720p (but not record) and they also record DVCAM.

I think it's necessary to make a list of needs (both shooting and deliveryi) and see how the different camera options working with the HD100, and the available editing options, fit best. (If you've been editing 720p and add 1080i, you might need a more powerful computer than you've been using, etc.)

Another HD100 would obviously be the best mate. I have two and they are very easy to match with the simple swap of a SD card.

However, I've often thought of investing in a HVX200 because of its small size and versatility of frame rates. I think it would make a great "crash-cam" in place of an Eyemo for stuntwork.

One consideration has got to be compatible decks. The JVC deck won't play 1080i, but I think the Sony decks will play 720p (but not record) and they also record DVCAM.
Very good point. Hadn't thought of that!
I think it's necessary to make a list of needs (both shooting and deliveryi) and see how the different camera options working with the HD100, and the available editing options, fit best. (If you've been editing 720p and add 1080i, you might need a more powerful computer than you've been using, etc.)
Basically I'd like something handheld with image stabilization to add flexibility to my arsenal. I'm not sure how well the 1080i of the Z1u will scale to match the 720p of the HD100. Will motion look the same? In other words, would the 1080i scaled to 720p look like real 720p?

1080i and 720p have approximately the same temporal and spatial resolution, so they cross-convert rather well.