I actually never looked at it that way...but it is also unfortunate because Xbox Live has been around for 7 years, without any increases in the friend limit. Between Gears of War games, Halo, the many multiplayer arcade titles, Call of Duty, etc...when you look at it on a per-game basis, 100 friends is VERY limiting.
Okay, that must be due to the short delay that the preview had going public. Still - keep your eyes on your inbox, and relax over the weekend ;)
TECHNICALLY, a little more than a few more days, unless you get into the Major Nelson Preview. Notifications for acceptances for that program go out today though, so watch your inbox if you applied. ;)
Yup, GFWL utilizes the same friend list, unfortunately :(