Updates definitely make a difference. Boy Wonder is exactly right -- you do want your name to be recognized (in a good way, preferably!).
Here's what happens when you're on the waitlist (at least, this is what my office did). Your file, along with the 100 other wait-listed candidates, is put in a filing cabinet in the administrator's office. Every few weeks, the top portion of the files (if they're ranked) or all the files (if they're not) are pulled out for the adcomm meeting. At this point, they're ruthless -- they have to whittle down the pile, and they do this by disposing of everybody who they don't immediately recognize -- who wants to have to read the entire file again!
Here's how to stand out -- write letters, updates on your grades, new transcripts, new letters of recommendations, whatever you can. When the administrator receives the letter, s/he circulates it around to all the adcomm members before putting it into the file. If you do a letter every 2-3 weeks, you're going to be at the top of their minds. When they get to your file (which by the way is now bigger and even more impressive), they think, "Oh yeah, I remember Moops, let's not get rid of him yet." Or better yet, they think, "Hey, he really cares about getting in here. That's the kind of med student we want!"
As for people who don't submit their final grades, if the school's have a tracking system like we had, one of the boxes that has to be checked off is "Final transcripts submitted." Some may slip through without it, but it should set off alarms.
Keep in touch with your wait-listed school and good luck!