Originally Posted by WiredGuy

Yahoo API is limited to 5000 requests per day (up to 50 results/request) and Google's API is 1000 per day (up to 10 results/request), *I believe*, I could be off.

Doing direct searches on the engine (not via API) is even less. Google will OCR your results, but its still useable, Yahoo will just give you an error and you have to wait it out.

Results and throttling is IP specific, yes.
WG