It depends on the theme. Not all themes seem to work well with blackberries, and to accommodate them properly I expect would take changes to the underlying theme files as well as specific modifications to the CSS. Wide images can cause blackberries to choke in my experience. I wish RIM would makes some changes to their browser as I've taken to avoiding the web unless I have to with mine, and I'm actually thinking of simply going back to my old Razr and downgrading my service. If I need to do web stuff, I'll just wait till I can get to a wi-fi hotspot and use my iPod Touch which seldom seems to choke on anything web related.
Perhaps in the "themes blitz" wordpress.COM has promising for this year, they will come out with a theme for the non-iPhone/iPod Touch handhelds.
If I were you, I would send in a request directly to staff so that they can log your request as we never know when, or how often they hit the forums, or if they will log a request here. Also, if you know of a wordpress theme that works with blackberries, make sure and include the name(s). Do be aware that wordpress.COM will only consider full-GPL licensed themes.