Usability 
Yelp.com is a site featuring user generated reviews of businesses, services and attractions mainly in major US cities. Restaurants seem to be the main focus but users are free to review anything. There are plenty of reviews of clubs, hair salons, auto repair places and even cell phone stores. I'd never heard of Yelp until I read Oliver Starr's writeup of it's new mobile site. Apparently I don't get out enough. It seems that Yelp is a genuine hit especially in the San Francisco Bay area where the site is based. There are close to 100,000 reviews posted on the site. The San Francisco restaurant coverage is very complete and includes most of the neighborhood favorites that you'll never find in tourist guides. Many reviews feature photos of the thing being reviewed including, in the case of restaurants, the food. A Yahoo map pinpoints the location of every review subject. In addition to San Francisco, Yelp seems to be taking off all around the country but especially in Chicago, New York and Los Angeles.
Besides being a review site Yelp is also a bit of a community as well. Yelp members can participate in a chat board and send private messages to each other. Many of the reviews comment on other reviews with the original reviewer commenting back. Each reviewer also gets a profile page where they can post their photo and list their likes, dislikes and a little biographic information. You can also nominate yourself for Elite status which if you're approved gets you invited to Yelp sponsored parties and concerts. Approval seems to be tied to being a prolific reviewer who participates in the social features.
Read more about Yelp, at Wikipedia and the The San Francisco Chronicle.
Yelp Mobile is sponsored by Palm which I think is great. Handset makers need good mobile sites to showcase their products. I hope this becomes a trend with Nokia, Motorola, etc. helping to support the development of useful and compelling mobile services like Yelp.
The Yelp mobile site contains all the Yelp reviews searchable by keyword and neighborhood or by distance from a city, zip-code or street address. Reviews are also browsable by category. It's not possible to log into Yelp from the mobile pages so none of the social features are available. This is probably just as well as the quality of writing in the current crop of reviews is quite high. Mobile posting doesn't lend itself in in-depth or even insightful reviewing. I could see the the messaging and chat being extended to mobile but so far Yelp mobile is strictly a review site. As such, it's a great thing to have handy when you are out and get the urge for some food, drink or entertainment and as a shopping resource.
Although Yelp Mobile claims to be "Optimized for Palm Treo" it works well on most any WAP2 phone. The reviews are text only and broken into pages under approximately 10 KB.
Photos and maps are on separate pages. The photos are 100x100 PX, 3KB each and are displayed 10 to a page. Maps are 240x320px. That makes both the maps and the photo pages marginal for some non-smartphones. Opera Mini is a good solution if your phone's browser acts up with Yelp's graphical pages. Or you can confine yourself to the reviews which are text and work fine on any device.
In keeping with the touchscreen Palm optimization, there are no accesskeys but that really isn't much of an issue as the site is well sized and organized for mobile use even with a keypad.