The BJCP organizes milds under their Style #11, English Brown Ale, with 11A designated mild and two additional sub-styles, northern and southern English brown ale. They suggest IBUs of 10-25, SRM of 12-25, and ABV of 2.8-4.5%, with most falling between 3.1-3.8%. The only American example they list is Goose Island PMD Mild.
For the 2007 Great American Beer Festival, The Brewers Association will for the first time include milds as a separate category in the style guidelines. Two categories, actually, as they’re dividing them into two separate styles, English-Style Pale Mild Ale and English-Style Dark Mild Ale. These appear to be the same as Category 56 for the World Beer Cup. For the BA, Pale Mild has IBUs of 10-20, SRM of 8-17 and ABV of 3.2-4% and for Dark Mild, IBUs of 10-24, SRM of 17-34 and ABV of 3.2-4%.
The color distinctions between the two are described as “golden to amber” for pale mild and “deep copper to dark brown (often with a red tint)” for dark mild. Both are dominated by malt favors with very low bittering discernible and allow for some low levels of diacetyl (butterscotch). The dark mild may also include some licorice or “roast malt tones.”
I imagine I’ll be loosely following these guidelines with my selection rather than drinking an "authentic" Mild—only because I can’t seem to find a Mild around here anywhere, short of brewing my own. And I didn’t have time to do that.
That’s okay, I can be creative and come up with something…