Yes or No 2
Aside from the benefit to users of the software, it is important that Apple follow their own guidelines for dialog design to set a good example for other developers.
Microsoft Word 2004
Saving a formatted document as plain text:

With the filename (if the document has already been saved) and an action-labeled button:

Firefox
Adding a login/password combo to the internal keychain (yet another reason I don’t use Firefox as my default browser):

Camino (among other OS X browsers) already provides a properly worded prompt for this:

The “Remember this password…” question should be bold. Off to Bugzilla I go…
Opera
Adding a syndication feed:

With proper button alignment, text styling, and an action-labeled button:
Gizmo
Prompting to add a person who has added you to their Contact List to yours:

Compacted, with action-labeled buttons (Do Not Add is used instead of Cancel because the dialog’s appearance is not caused by direct user action):

TextWrangler
Prompting to install the TextWrangler command line tool:

Prompting to install the twdiff command line tool:

A single dialog with action-labeled buttons (again, Cancel is not used because this dialog is summoned not by direct user action but automatically when the application is first launched):

This dialog provides a bit of information about how to use the tools and can scale reasonably well if additional command line tools are included in a future release. The buttons for installing tools later should be in one Preference window section.
ecto
Resetting the application and renewing the trial period:

With slightly reworded text and an action-labeled button:

iTunes
The dialog to confirm that an image should be embedded in the tracks of multiple selected albums can’t decide whether it wants to be a Cancel/OK or No/Yes, so it splits the difference:

With a clearer explanation of what will actually happen and an action-labeled button:

Dialog wording and button labeling is very important to users because poor design can lead to inadvertent data loss.


So what exactly would happen if I unchecked both checkboxes on the TextWrangler dialog then hit Install…
Other examples are good. I think the laziness of Yes/No OK/Cancel come from developers used to other UI toolkits that provided dialogs with the buttons already set.
[)amien
Damien:
Admittedly, I hadn’t thought of that! The Install button would become unclickable if neither box was checked. I think most people would just click Do Not Install if they didn’t want any of the tools, but handling the scenario you mention would be necessary.
I was just about to post what Damien did. In addition, what happens if you untick *one* option and then click “Do Not Install”? Will it still install the other one? I’d say that “Do Not Install” actually should be “Cancel” even though it isn’t user-produced, to avoid both of these confusing elements (and I agree that “Install” should be disabled if both are unticked).