I tried the seeds. Your seed 100 is cool (many more caves than I thought) and if you go to seed 99, you get a huge canyon at spawn perfect for castles overlooking the canyon. I found no reasonable flat area, except about a 10x30 size at most (with trees).
They will always have trees... and sometimes it depends on what you mean by flat.
Now that I've built a farm, I notice flat land a lot more than I did before.

The larger the body of land, the more likely that there will be some larger flat areas... at least in my experience.
I haven't tried seed 99 before... now I'm really tempted.

When there is a UI for entering "seeds" it will be more fanciful. I can already see that I'm going to have to provide a way for users to export their world's seeds for others...