Further to my earlier comments, my initial thoughts seem to be holding up. I.e. You should not be developing separately for mobile devices. The larger screen sizes, and the fact that 99.9% of the web is written in HTML, seems to be making XML redundant. What you need to do is:
Decide on the minimum screen width you are going to design for. I suggest 320px.
Decide whether you are going to support Symbian devices. There are bilions of them out there, but they are little used on the web. E.g. Latest global web volumes from w3counter:
Apple iOS 5.66%
Android 1.81%
BlackBerry 0.57%
SymbianOS 0.18%
Total 8.22%
The total mobile presence on the web is less than 10% (but growing fast) of which Symbian devices account for less than 0.2% (and falling). So it's time to start taking mobile seriously, if you are not already. It seems that iOS and Android are the "must support", BlackBerry a niche market, but Symbian on the way out?
If you do decide to support Symbian devices, you need an actual Symbian phone to test on, to see what works, and what doesn't. I use one that connects via WiFi (it's a lot cheaper that way!).
Test your site on the w3c mobile validator. I've got one site up to 70% mobile compatible, just written in HTML 4 + CSS 2, and can improve on that still...
The main constraints seem to be:
a) Do not rely on JavaScript for essential functionality.
b) Animation is a problem, with Apple chosing not to support Flash. The best compromise in the short term may be old-fashioned GIF animations. But even they may not work 100%. E.g. GIFs loop on my Symbian device when they should not, and the timing is ignored on the Kindle!
c) You can write mobile friendly sites in HTML and CSS, but only a sub-set of the languages may be supported. E.g. Iframes may not. So keep it simple.
d) Use fluid design techniques to allow the pages to contract down to 320px and expand up to 800px, or so.
e) Think small. Ideally your web pages should be less than 20K Bytes (including graphics)! That's pretty horrendous and, thankfully, increasingly irrelevant as it relates to the 2G networks. But it's a target to aim for. Size matters, particularly the initial loading of a page. Even 3G is not particularly fast (by broadband standards).
