In Europe and here in the UK mobile use is huge, and the market place for pushing apps and traditional sites onto the mobile platform is serious business.
I for one would welcome the ability to have a usable theme for UBB on my iPhone (other mobile platforms are available :))
I find trying to access my site via mobile to be clunky at best; and being on the move a lot there is a regular need to be able to do this for moderation / post approval needs. I can't always rely on my nominated Mods being available to do these tasks for me.
I dont think data use/bandwidth issues enter into it over here. Most providers include an allocation for data use now, even if several providers have had to "re-word" their fair-usage terms.
Just my two-penneth worth to chip into the discussion