I'm still grandfathered in with AT&T unlimited too since I got my 3GS on launch day back in June 09.
This whole throttling thing has me concerned too, ever since AT&T must've added a new tower in my area - Grand Rapids, MI - back in the spring I've been getting full 3G bars at home, whereas up until then I really didn't get a 3G signal at home. However I've got WiFi at home all day and night, so it didn't matter. But not I find myself using 3G a lot more now, especially if I'm downloading and/or streaming content from my laptop or my PS 3 over WiFi. Or sometimes, after getting home from work, I simply forget to turn 3G off and turn WiFi back on. I used to keep 3G off at home, since I have WiFi, and then turn 3G on whenever I leave the house.
Whereas I used to trend around 1.5GB of data use per month, over the past year, I have been trending 4-4.5GB per month, over the last few months. I honestly can't see myself going over 5GB per month however, so I'm HOPING to not fall into that top 5%.
Although, seeing the usage amounts that others have posted in this very thread, of people who have used over 10GB of data per month, and knowing that I haven't even made half of that amount, makes me feel a bit better about things for myself.