Checking IPs is an iffy solution at best. Since HTTP is essentially stateless, they can't prove that anyone individual is necessarily guilty of anything. However, they recently requested that a module be removed from the CPAN for violating their terms of service. I think that's fair. I think as a matter of being a good netizen, it's appropriate to respect their restrictions.

Here's the relevant section from their terms of service (which is why you should use their SOAP interface - it's really easy).

You may not send automated queries of any sort to Google's system without express permission in advance from Google. Note that "sending automated queries" includes, among other things:

• using any software which sends queries to Google to determine how a website or webpage "ranks" on Google for various queries;
• "meta-searching" Google; and
• performing "offline" searches on Google.

Please do not write to Google to request permission to "meta-search" Google for a research project, as such requests will not be granted.

Cheers,
Ovid

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More generally, sites can analyze the "signature" of requests to identify if they want to block you or not. By signature i mean anything that can make your requests stand out from those of the other 99% of their traffic. They might do it based on your User-Agent, or some other HTTP header that is unique to the API you are using, or they might do it based on some combination of things that help identify people who are being decieptful (if your User-Agent says you're Netscape 6, but you use "HTTP/1.0", that's a dead give away ... other more subtle things might be descrepencies in what HTTP headers you send vs. the headers that Netscape 6 ALLWAYS sends.

Bottom line: play nice. If you get blocked, you probably deserved it.