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JS Fiddle

JS Fiddle

tl;dr

JS Fiddle

TL;DR Based on an informal usability study conducted on a similar need (but in a different problem domain):

JS Fiddle

1. Put the "none" option and the other options in a side-by-side list. Put the most common option in the first read position. So, if the language is L-R and the most common option is "none", place "none" in the list on the left and the other items in a list to its right.
2. The "none" should be a checkbox, like all the others
3. The checkboxes should act as a toggle: if "none" selected, others are disabled. If any one of others is selected, "none" disabled.
4. Use text color/font weight/font style to separate "none" from the others.

1. A long list of the options with a "none of these" radio at the bottom.
2. Same as 1, but with a "none of these" toggle button instead of a radio.
3. A long list, but with a "none of these" radio offset out of the flow.
4. Same as 3, but with a button.

The control, and original implementation, was #1. All scenarios were presented with similar formatting: same font, same line spacing, etc. If the "none" option was selected, there was no "graying of the list". (Pop stats: 62 users, avg. age 37, 59% female, 5% experience a form of color blindness.)

1. Separate the "none of the other options" option from the list of other options.
2. Place the two lists side-by-side, such that the most common list comes first.
3. The "none of the other options" option should be a check box, like all the others.
4. The checkboxes should act as a toggle: if "none" selected, others are disabled. If any one of others is selected, "none" disabled.
5. Use text color/font weight/font style to separate "none" from the others.

1. A long list of the options with a "none of these" checkbox at the bottom.
2. Same as 1, but with a "none of these" toggle button instead of a checkbox.
3. A long list, but with a "none of these" checkbox offset out of the flow.
4. Same as 3, but the button is offset.

The control, and original implementation, was #1. All scenarios were presented with similar formatting: same font, same line spacing, etc. If the "none" option was selected, there was no "graying of the list". (If you care, the population stats: 62 users, avg. age 37, 59% reported as female, 5% reported experiencing a form of color blindness.)