We've been posting our data type plans on the data type blog for quite a while now, but the blog format hasn't worked well for comments and discussions. We've therefore created a new data type forum that will contain discussion threads for each of the data type posts. It can also be used for other questions about data types.
It's hosted on the new 3.0 forums software, which means it looks a little different than the main forum. The main forum will migrate to the new software in the near future.