There's quite a difference between the terms of use of Google's and Yahoo!'s search APIs. While both say you're not allowed to do anything illegal or mission critical (I especially like Yahoo! saying you can't rely on the API if you're operating nuclear facilities), they differ in terms of what you're allowed to do with the results.

Google spends a lot of time saying what you're not allowed to do once you get results (you can only retrieve a small number of results (8 at the moment), you can't use them as the main content on your site, you can't modify them, you can't use a robot or spider to retrieve results). Possibly fair and reasonable, though, for my current (non-commercial, research) project, 8 results won't do and I want to break all those rules.

Yahoo! on the other hand, says absolutely nothing on the subject of what you can't do with the results. All they say is don't whack the server too much.