I have looked at PhoneGap and Mono/MonoTouch along with a few others.

I think you still need to know how to develop a native app for a platform even if you use one of those - tweeking is going to be necessary to make them truly a native app.

I've built a couple of WP7 native apps so far (using Silverlight and ESRI for mapping). They work fairly well considering the UI area is a little larger than a post-it note. I've handed stacks of post-it notes to all of our project managers and told them to make sure they design UI that fits on there before selling a project to a customer or coming to me to make it happen.

The ASP.NET site I'm doing for the iPad is a convenience since the app is supposed to work on iPad and a full size browser. I do make iPad seperate friendly screens on things that involve any kind of data entry. So far the results are pretty well received.

>Most modern phone browsers will work to read Web Pages - the question is how well depending on the size of the design usually. I can browse just about anything on my Windows phone, but that doesn't mean it's pleasant.
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>To design apps that work well on mobile devices you really need to take into account the a) size, and the b) the touch interface. If you want to build an app that feels more integrated into a device touch specific gestures are fairly important. YOu can use mobile frameworks like jquery.mobile and various others.
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>In addition there are several tools out there like PhoneGap, Titanium etc. that provide a unified feature API using javascript for core mobile OS features (like touch gestures, GPS access, access to dialer/contacts etc.) but use plain HTML for design. THese tools then have mapping compilers that compile that HTML down to native binaries for the device.
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>I've fiddled around with PhoneGap a bit and it works fairly well although the apps it creates on devices don't feel quite like true native apps. Still pretty good though considering it offers a fair amount of cross-platform functionality.
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>All this is so clearly in the early stages through and it's all bound to change drastically in the next few years so unless you have an immediate need I wouldn't waste too much time getting proficient just yet. Things will change rapidly in this space and todays tech will be left behind quickly I think. IMHO HTML5/CSS will be the future with this stuff as vendors figure out that developers will go to more general platforms that work across devices. Right now we're in that 'hype' phase where 'gotta have an app' rules even when that often doesn't make all that much sense.
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>We'll see - I think the next couple of years will be really interesting in this space.
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>+++ Rick ---
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>>>>>If you've separated the app into tiers nicely, you can wrap the back end in web services and rewrite the UI in HTML5/JavaScript, which can then compile into most mobile devices.
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>>>>In other words, no :)
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>>>>You also need to determine if the application requires local storage - this ups the challenge factor.
>>>>
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>>>But suppose the application does not require a local storage and all data processing is done on the server, do you think ASP.NET with HTML5 is a good approach for, say iPad?
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>>Yes. The iPad browser (safari) seems to render standard asp.net sites pretty well. Not mac pretty, but it works.
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>>I'm actually doing one now that is ASP.NET that will (hopefully) handle IE and firefox desktop along with iPad. I'll be doing a few iPad specific pages that will reduce what is displayed, but for the most part it will be common pages for the real world and iPads.