Discount travel site Kayak has launched a new version of its popular, free iPhone app. The new app, which is already in App Store, includes a trips management tool, an upgraded user interface and faster search.

New features and upgrades include a two-dimensional desktop-style navigation system, an enhanced flight, hotel and car search design including a new scrolling calendar, faster search and additional search options and filters. It basically comes one step closer to bringing the web experience of searching Kayak to the iPhone. The app also serves as an itinerary management service that consolidates flight, hotel, car rental, maps and other events into a single itinerary. Kayak’s paid app ($3.99) for flights on business or first class has also been updated with the trip management feature and improved search and UI.

One feature missing from the app is the ability to conduct a transaction within Kayak; if you purchase a flight or hotel, you are led to the airline or hotel chain’s mobile page, which is often difficult to use on the phone. To remedy this problem, Kayak plans to add a booking assistant tool to its next generation of iPhone and mobile apps in 2010 that would allow users to buy the travel item without having to go the the airline, hotel or car rental site. Kayak will also be folding in advertising into apps but isn’t sure how ads will be integrated.

Kayak is continuing to expand its mobile strategy, with several iPhone apps, a BlackBerry app and and a recently launched Android app. The iPhone app is amounting to 5 percent of Kayak’s total search queries (around one million per day), and already has 600,00 downloads after launching in February. Kayak is pouring 20 percent of its development resources into mobile, says Kayak’s CMO Robert Birge.

Kayak, which has raised a healthy $223 million in funding, has seen steady growth in a over-crowded space. And Microsoft’s Bing newly launched travel search engine looks a whole lot like Kayak’s but isn’t imitation the sincerest form of flattery?