As someone with a blackberry that's been looking for something like this for a while, I vote mobile site since blackberry is usually the last one to get an app

As someone with a blackberry that's been looking for something like this for a while, I vote mobile site since blackberry is usually the last one to get an app :) But seriously, if you're not looking to spend a lot of time on it I'd go with the mobile site option. It works for everyone right away, is going to be the easiest to implement (basics only, of course) short term, and you can gauge what the demand is like for mobile use.