The first thing we need to ensure is that we have the legal right to use information from other search engines. We also need to realize which are open-source products, as these will be more prone to changes. This means that it is a good idea to have some sort of logging and testing to ensure that this service remains available.
We are going to be looking at the integration of Google's site search features. We have integrated the site features into a custom tag to make things easier to implement. This is just a prototype of a full-featured work that we will make available later. Check my blog for details and other works.