If you find someone that has it, they can give you the IPA file, and it will work. If you have an iPod touch 1G, though, you can still jailbreak 3.1.1, and then use the latest version of Yelp. I think Yelp requires 3.1 or 3.1.1 in the latest version because it uses new camera API's that weren't published in 3.0.
Or if you really wanted, you could get an app from Cydia that changes the version number as it appears to apps like Yelp. You could "change" the version number (doesn't upgrade software, just fakes it) to 3.1.1, and Yelp may still work. If you really don't want to upgrade and can't find a Yelp 3.0.0 IPA, it's worth a try.