With Zen, you can control which five icons appear on home screen. They're the first five icons when you press the menu button. Just arrange them like you want them.
With Today, the icons that appear are locked, but the advantage is that the system will give a preview of each one (messages, calendar entries).
I don't know that any one would be considered "standard". I got my first bb about 10 months ago, and I'm thinking that the AT&T theme that first showed was an icon one, but I'm not sure. My personal preference is a zen, since I can see the most used icons along with a good wallpaper image.
I like the Zen as well, because you can differentiate between email accounts/icons by hovering over the icon. So, I can easily tell whether the new email I just received is a work email or a friend sending another stupid joke to my gmail account.
I use the Icon theme simply because it's the only one I could use to wallpaper the whole screen. The other themes kept the AT&T stuff at the top and the wallpaper was sort of cut off.
I arranged my Icons into 3 folders - applications, messages, and utilities (ok and a hidden folder with crap I don't use) - so that's all that's on my home screen - ok along with weather eye icon and the screen lock icon for quick access. I just change the color of my folders to match my wallpaper.
so would the standard verizon one be considered a "zen"?
I'd say that it's a variation of a "zen" theme because you can customize the 5 icons across the bottom and then just hit the menu button once, and all of your icons will fill the screen. I have a verizon curve, and I actually like the "verizon" theme better than the others. (Hehe, imagine that - someone actually LIKES something that verizon does. Who could have guessed?)