I'm curious as to how we could force a manual update without the servers pushing it. A quick look at the source brings up this:

Code:
        // This Extra must always be set by the caller.
        File update = new File(intent.getStringExtra("updateFile"));
        if (!update.exists()) {
            // The update has gone away, maybe replaced by something newer?
            Log.i(TAG, "OTA update no longer exists: " + update);
            finish();
            return;
        }

        Log.i(TAG, "OTA update available: " + update);
        Uri uri = Uri.fromFile(update);
        Intent install = new Intent("android.server.checkin.FOTA_INSTALL", uri);
        mInstallIntent = PendingIntent.getBroadcast(this, 0, install, 0);

        long now = SystemClock.elapsedRealtime();
        long nextPrompt = getNextPromptTime(intent, now);
        if (nextPrompt == 0) {
            Log.i(TAG, "Installing overdue OTA update without prompting");
            installUpdate();
            return;
        }
This implies that both:
a) the URI for the update is stored in the file "updateFile" somewhere on the device, and
b) that Google has the ability to install OTA updates WITHOUT the user accepting.

a would mean that forcing an update would be rather simple, but as I have not received the update yet, the file shouldn't exist. Can anyone here that has debug access to their G1 (I haven't set this up yet, so I'm unsure on how much control you get from this) post the contents of "updateFile"?

Edit: I'm stupid, I read that wrong. The file WON'T be called updateFile, but rather that is just a variable in the intent that contains the actual file location. None the less, if you cancel the OTA update, the file should be fairly easy to find.