Hi fourroses666,
There are a number of options, depending on what you're looking at doing with it.
1) Regular website with Responsive Web Design: The line between what is a web site and what is an application gets narrower every day, and if your application can be done as a web site, then this is probably the best way
2) Phone gap:
http://phonegap.com This is basically to take a web site and wrap it in an API for each platform you want
3) Per platform O/S with MODX serving up XML for resources: One of the reasons that MODX calls resources "resources" instead of web pages or documents is that MODX can serve up anything, including any kind of communication.
As for my personal preference, I personally do 1, but would love to look at option 2 as well.
In terms of Option 3, I don't like the idea of maintaining multiple code bases, especially if the application is doing complex things which require custom coding in each item. If you're not doing something that is important enough to throw a bunch of resources at it for development and then ongoing for maintenance, I'd stick with options 1 or 2.
Hope this helps.
Regards,
Tom