I've looked through all the RIM/blackberryfaq doc and I still cant figure it out. Basically, my trackball fell out, so I need to use shortcuts. But I cant how to scroll up/down a list. In other words, I do ALT-S to get to the message list, but all I can see to do is scroll up or down by page or date or first unread message.
Is there a way to simply scroll down to whichever message I want?

Space bar.

Space bar pages down. I want to get to a particular message.

N moves to next date
P moves to previous date
U moves to next unopened item
J moves to next related message
K moves to previous related message

hope that helps!

Well, not really...

If I do "m", then alt-S, it takes me to all of my SMS messages. All of my messages are "read", so how I would I get to, say, the 4th message down in my list? I'm guessing at this point there isnt an easy way...

Nope. No way to do that. You will need to get a new trackball first.