This is a story about one of my worst decisions as product manager for OmniGraffle.
A few years back, it was brought to our attention that the keyboard shortcuts that we were using for "Zoom In" and "Zoom Out" were also being used by Apple as part of the Seeing portion of Universal Access. It was a recent change, and so we decided to avoid any conflict by assigning a new set of shortcuts for zooming. Turns out, the new combos we chose were *also* used by Universal Access, for the very same functions, but only when the new aluminum keyboard was used. So, we had to scrap the new shortcuts and start over anew.
Side note: Many people argue in favor of using ⌘+ and ⌘- for zooming in and out, they are after all the Adobe shortcuts and have been for years. They are also Apple's default for making text larger and smaller, and we try not to conflict with what Apple has decided should be the system standard (as opposed to the de facto standard, consider ⌘G and ⌘U for group and ungroup, whereas Apple uses these for Find Again and Underline, respectively).
One can, of course, change these shortcuts on a global or per-application basis in System Preferences to what is desired, and I encourage doing so.
So, we went with Option-Command-Plus and Option-Command-Minus, which didn't work out, so the Shift key got involved (since Plus is Shift-Equals anyways), and we wound up with an unwieldy chord of ⌥⇧⌘= and ⌥⇧⌘-, because that's what the menus wind up displaying in the end. The bad decision was actually to pick a combination that Apple was unlikely to ever use, since there are many other ways to zoom in and out in OmniGraffle. For instance, you can use the Zoom Tool, or the Zoom Popup in the lower right of the window, or hold down Command and use the mouse's scroll wheel, or pinch to zoom using a trackpad, or press the Z key and click with the mouse.
With all of those options available, the keyboard shortcut that we arrived at has led to more complaints than I care to number.
We've very recently changed these keyboard shortcuts to match the iWork apps (⌘> for Zoom In, and ⌘< for Zoom Out) in the forthcoming OmniGraffle 5.4 release. This likely won't appease people that are accustomed to and wish for the Adobe shortcuts, but it's a better solution than the one that is in place, and as stated above, they can always be changed if needed.