If I have Facebook mobile downloaded onto Blackberry, but go on Facebook on a computer... will I be able to see the activity (from the computer) on my Blackberry?

I hope the above question makes sense!

Yes, you can see that activity on your BB and any activity you do on your BB you can see on your computer.

so what exactly shows up? Is it mirroring each other exactly?

The Facebook application doesn't mirror exactly, as it doesn't show updates. But from the application, you can go to Facebook Mobile and for the most part, you'll see all the updates.

but it does show you any messages, wall entries, pokes etc... that have been posted to your facebook site and allows you to post to others in the same manner. Thus it is a pretty good post box service for facebook.