I looked all over in Mail to no avail. Thanks to this Apple discussions forum post, though, I found the answer! On the bottom of the Keyboard Shortcuts tab in the Keyboard and Mouse System Preferences panel, click the radio button next to "Text boxes and lists only" from "All Controls." Warning: This is a system-wide prefences settings, so tabbing in Safari, etc. will also be affected.
[robg adds: I do exactly the opposite to any new machine -- make sure this setting is on All Controls. Even though it costs a few extra keystrokes here and there, it makes the keyboard much more useful in every application.]

