Yahoo ban info at our Search Engine Watch Forums covers some tips on getting back into Yahoo if you've been removed for some reason, while Why is this site banned from google? touches on getting back in Google's good graces -- or at least how to ask. In summary:
- Yahoo: Send to ystfeedback @ yahoo.com (remove those spaces, obviously). Alternatively, you can try the online support
- Google: The old email request system no longer works -- I just double-checked that. Instead, use the Google online support form specifically for webmasters.
Before requesting any reinclusion, it's always best to ensure that you've sorted out any problems you already know that your site has. What types of problems might those be? Read the manuals.