It is important to keep some things in mind when you decide to make your desktop web app go mobile as you will have more use cases for your app to deal with when on mobile.
So is mobile web just a stripped down version of classic web?
Its true that you wont be able to bring on everything you offer on the big web on mobile coz of reasons like small and varied screen sizes and lower bandwidths etc. But that doesn't mean your mobile app can be with limited features. When you design for mobile keep in mind that your users are on the move and what feature would make sense to be offered to such a user. If you know the answer then try your best to make them available on mobile no matter what screen size and bandwidth. What you need to do is to think of how optimally you can do it and not get bogged down by the restrictions like screen sizes, low memory, inconsistent browser behaviors etc. Thats when you will think of new formats to present your screens. Like how they have different ad formats for tv, radio, print and web, there are desktop web and a mobile web UI formats. New format but same feature.
My favorite travel app cleartrip recently went mobile and their UI follows the typical cleartrip design philosophy of clean and simple; a format which works hands down on mobile too. The format works well but feature wise they are not complete; as of now they allow only one passenger to do booking in one shot. May be its just a matter of time but I think there is no reason to believe that on mobile the user will only book tickets for himself.
Think out of the browser
Once you have got the format and the features working well start thinking out of the browser. Mobiles are not only internet devices but also other things. Yeah I know I'm re-iterating the now old statement "a mobile is not only a phone but also an internet device". Think how you can make use of the other capabilities of the phone. The phone already has a community built in - the address book. The phone has voice, text , calendar, camera, video, audio and the big one, user's location. Try to see if your features can be enhanced meaningfully using these capabilities of the phone. Twitter is one service which has made use of mobile capabilities to enhance their service.
Take it or not, the primary way someone interacts with a phone is by making calls. Hence its important to integrate click to call features wherever possible on your app. I also think this is one of those features which is only possible on mobile. Cleartrip has a "call to book" link on every search result which is a very good example of how one can think out of the browser and integrate native capabilities. However they don't offer a way to call them from their home page and hopes everyone to first do a search and then call them only to make a booking. What if I wanted to call them for some information or something else other than booking? Or what if I just wanted to call them and make a booking directly rather than making a search for just one passenger?
Think user behavior
Everything related to mobile is about multi tasking. And when you multi task, you snack. When you multi task your attention span is low. Its important that all mobile apps understand this point and make the screens straight to the point and focussed. Give information in small chunks. Give absolutely no margin for errors.
The phone is always with you, even when you sleep. Mobile services will have the luxury to be in touch with the user all the time. But as the phone tends to be very emotionally attached to the user, the reactions from him can be of extreme nature. You reach up to him just when he is need of you otherwise he will quickly label you as spam. Even if he makes a mistake he will blame you for it.
I think the most easiest form of user interaction with mobile is by voice. Android is in the right direction as they are coming up with voice actions. I would have loved to have an alarm on/off/snooze voice action and a call becoming active the moment i say "hello". I know that we haven't yet mastered the science of voice inputs but for now make the users type less at least.
Overall see if your service will help someone on the road. If not its better not to even attempt it. But believe me its all about your strategy on being mobile.