jQTouch is a plugin for jQuery that allows you to build mobile websites for the iPhone and iPod Touch. There are plans to also support Google Android and Paml WebOS browsers in the future. jQTouch was written by David Kaneda and is completely open source and MIT licensed. If you are familiar with the iUi Framework, jQTouch should look very familiar. It shares many of the same conventions but packs a lot of new features and leverages recent advancements in the Mobile Safari browser.
The markup for a jQTouch site is pretty simple. Each "page" on your site is a div with an ID in the body of the document. Links point to the id of the div you want to show, and jQTouch automatically transitions the view using a slick animation. jQTouch allows you to easily create native looking elements like buttons, links, and forms using very simple html structures and CSS classes. Pages can be loaded via AJAX as well. Simply put a url as the href for a link and jQTouch will automatically fetch and load your page. AJAX pages should just be the HTML fragment that contains your page, not a full HTML document.
Not every website needs a mobile version, because modern smart phones and other mobile devices are capable of making normal websites look good. However, if the site you're designing is content heavy or if it's for a brick and mortar business, a customized mobile experience is worth considering. The reason I've singled out these two cases, is because they're commonly accessed by people on the go (I'm drawing on personal experience, here). If a user is browsing the web while waiting in line at the cinema for example, content sites are a first choice. Additionally, if a user is looking for a nearby bookstore or restaurant, it's important that they're able to easily access information like addresses, phone numbers, and menus.
There are a lot of considerations when designing for mobile devices, but the most obvious is the limited screen real estate. Designing for the small screen can be a challenge for beginners, and it's still a relatively new area of design, but the easiest thing to do is to cut out as many elements as possible. For most websites, there's no way you'll be able to include all the features in a mobile version.
Another interesting feature of newer mobile devices is that many of them support for the Geolocation API in HTML5. This means that, with the user's permission, you can use their current location to present them with more relevant information. For example, if you're designing a site for a restaurant chain, you could provide the user with 3 nearby locations or maybe some discounts that are exclusive to locals.
Most mobile devices don't support Flash yet, notably the iPhone, so mobile websites should avoid this. Restaurants, band websites, and other consumer facing sites tend to gravitate towards Flash and other browser plugins, which is to be expected. This isn't OK when there isn't a mobile version that's free of these plug-ins, however, which is all too often the case.
Additionally, many mobile websites don't support the same URL structure as their desktop companions. For example, when tapping a link to a video that's on College Humor, the mobile site redirects you to an index of videos. I consider this to be bad practice. Why not just go straight to the link the user wanted in the first place?