The UFC 100 weigh-ins starts at 7PM ET on Friday at Las Vegas, you can watch the ufc 100 weigh-ins online here are some sources, to watch ufc 100 online;

1. At FanHouse:
you need to install silverlight plugin to watch UFC 100 online from FanHouse.
Date Time: Jul-11-2009 7pm PT/10pm ET
Event Type: PPV Live
Location: Mandalay Bay Events Center, Nevada

2. Watch UFC 100 from UFC.com
Watch UFC 100 on Pay Per View (outside of North America, Click Here), and worldwide on Yahoo! Sports and UFC.com and UK.UFC.com.

UFC heavyweight champion Brock Lesnar

UFC 100 fight card
Brock Lesnar vs. Frank Mir
Georges St. Pierre vs. Thiago Alves
Jon Fitch vs. Paulo Thiago
Michael Bisping vs. Dan Henderson
Yoshihiro Akiyama vs. Alan Belcher
Stephan Bonnar vs. Mark Coleman
Mac Danzig vs. Jim Miller
Jon Jones vs. Jake O'Brien
C.B. Dollaway vs. Tom Lawlor
Matt Grice vs. Shannon Gugerty
T.J. Grant vs. Dong Hyun Kim
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Today's Schedule
Mandalay Bay Events Center doors open at 3:45 pm PT
The first bout will begin at 4:35 pm PT
The first televised bout will begin at 7:00 pm PT
check out full schedule of UFC 100
Watch UFC 100 Live on Pay-Per-View July 11, 2009. Frank Mir spends some time at the pads during a Las Vegas workout in preparation for his bout against Brock Lesnar at UFC 100. Watch UFC 100 Live on Pay-Per-View July 11, 2009.

youtube video UFC 100: Frank Mir Pounds the Pads.