Strangely, neither the embiggened friend list cap nor the "Finder tab" are present in our preview version of the upcoming Dashboard update. We're still locked in to the smaller, more intimate, friend limit. We're checking with Microsoft now to determine when this update will roll out to you and the masses of people waiting to be muted by you.
Update: It appears that the "Finder tab" feature refers to the "friend finder" on the Xbox Facebook site; not a part of the Dashboard.
Update 2: Major Nelson tweets,"We will not be expanding the friends list as a part of the next Xbox Live Update." That's not to say the friends list cap won't eventually be tweaked -- merely that it's not one of the changes included in the pending fall update.
Original Facebook announcement (Xbox Germany):
Eine von vielen langjährigen Xbox LIVE Usern gewünschte Änderung war das Limit der 100 Freunde. Mit dem Update könnt ihr jetzt bis zu 1.000 Freunde eurer Freundesliste hinzufügen. Wie viele Freunde habt ihr? Nicht genug? Dann sucht euch neue über den Freunde-Finder-Reiter!
Translation (via Google):
"A lot of long-term change was desired by Xbox LIVE users the limit of 100 friends. With the update you can now up to 1,000 friends of your friends list. How many friends do you have? Not enough? Then you looking for new friends on the Finder tab!"
Screencap (translated version):