I too experience this problem (in all browsers). My guess is that we both followed the advice for a larger textarea as you describe above.

After some experimentation, I made the following changes to fix the problem:

textarea {
    width: 75%;
    height: 25em;
}
[download]

And through some quirk of fate, I added a lengthy url to my Personal Nodelet that forces the nodelet bar to be wider than normal.

Now I enjoy the Monastery without the annoying horizontal scroll bar you mention above, my chatterbox is a nice width for reading, and I still have a sufficiently large textarea for comfortable use.