I have done quite a lot of changes to the default MJ styles, mobile_pda and mobile_iphone and found out that mobile_pda now has changed to mobile_smartphone which is good but for my styles it caused some problems. For example the pathway to the header logo had to be changed from mobile_pda to mobile_smartphone.
Reagrding the styling changes: The mobile_smartphone/css/custom.css file and mobile_smartphone/css/mj_xhtml.css as well as mobile_smartphone/resources/styles/baseStyles.css are still almost the same except for some minor changes that caused me a few hours adjustments.
The summary however is that the upgrade is for the better indeed as smartphone is much more relevant than pda in my eyes.
I would still have loved some kind of styling for dummies here that tell syou which files (css) to play around with for changes of certain elements. Especially for iphone and smartphones.
Still using firebug anf firefox is a life saver and then useing the yourdomain.com/?device=iphone or smartphone helps a lot detecting the styling and changes that has to be done on the fly.