In previous versions of OS X, there was an option for laptops to ignore trackpad input when a mouse or other pointer device was connected. For some users, however, the option to disable the trackpad when a mouse is present is missing in OS X 10.5.6, and more still have noticed this feature gone after updating to 10.5.7.
In addition to this missing feature, the option to prevent accidental trackpad input when typing (as discussed in this Apple support article) seems to have been removed as well. Apple might be downsizing various options in OS X, but some of the decisions seem to be rather arbitrary, such as these and also the removal of locations in the battery system menu as we discussed in a past article on the 10.5.6 update. We're not sure why Apple would do this, but you can send feedback to Apple and suggest they include these features again in future updates.
Apple Feedback Pages:
In the past, people were able to manually change the trackpad inputs with the following commands; however, even these commands do not seem to be recognized by OS X 10.5.7.
There might be a hidden setting such as the "ignoreTrackpadIfMousePresent" string, which will enable or disable this option; however, we have not found one yet that will work with 10.5.7. One user "kemo" on the MacRumors message boards found that the trackpad preferences include the old software that does contain these options, and created a modded version of the system preference pane to use the old software.
Installing the mod will work to enable the missing options, but as other users reported, it may break some other trackpad-related functions and features, so we recommend you take caution if you want to try this, and be sure to back up your old trackpad preferences file before using new and modified versions.
We will be on the lookout for ways to enable these and other missing features in OS X, so if you have any tips or suggestions, please let us know and we will be sure to post them in future articles on this.
UPDATE: This appears to be specific to certain MacBook models. Currently it's not specific which ones, but it may be ones with multitouch-capable trackpads.Resources