It's been asked for many times (most recently here) and now Tracy and I are excited to announce that the auto-zipping of workbooks attached to posts is NO MORE!
Please feel free to test in the responses to this thread. I'm especially curious about you IE 8 users.
One small visual item that we'll smooth out later on is that .twbx files still show a zip icon as an attachment and a .twb's icon' looks like a text file. Once downloaded the files are fine, just left over icon references we're still working on.