Is this within the TOU for whois queries?

If you want to keep the queries mostly in-house, you can get access to Verisign's copy of the .com zone file for free and download it once a day.

http://www.verisign.com/domain-name-services/domain-informat...

And if you don't even want to change your code, you could fire up a DNS server that serves the entries from that file.

that's really clever, using DNS as a first-level filter and only going out to whois if that comes up blank.

Great product. Hope you make a tonne of cash! :)

What DNS record type are you looking up? If it's A rather than NS, you might want to rethink... NS will return responses when A might not.