Mobile Web Applications require an extra bit of awareness and effort from users on the platforms that have browsers good enough to utilize them. I haven't personally seen great adoption rates for them, but they do afford benefits development wise, especially if your goal is to create and launch a cross-platform application quickly. It's certainly a possibility that mobile web applications may get more user awareness with better discovery tools, but the general momentum of user interest has really centered on native applications. Mostly because users, that I have experience with, find using native applications a better experience to web applications, but also because the ecosystem around them has matured quite a bit to make finding and acquiring them so much easier.

There are a number of native applications that simply serve up mobile web applications and that grants them several of the advantages of native applications, though not all, from a discovery, capability and usability perspective. Users typically notice that their experience is sub-par compared to truly native applications, but good ones can get close enough that it doesn't make a huge impact compared to the benefits. You will of course still have to deal with app store/market place submission, but you could always launch the application as a web app until the native version is available, allowing you to even provide the native versions after your intended launch date. Generally speaking it affords you a lot of flexibility for your effort, but I would caution that HTML5 based applications really only work for focused tasks that do not need to rely on native features the varying phones provide.

The questions you really need to ask to make a solid decision on which path to take are, how many platforms are you really targeting and do they all need to be supported immediately? Are there any native features of the phones you're targeting that need/should to be leveraged to make your application useful and successful in accomplishing its primary task? How much effort are you willing to put into supporting and maintaining the application(s)? If all your answers lead to having to support a lot for a little and your application is simple enough to not need native features then an HTML5 implementation would probably make the most sense.