One of the advantages of ARC is the fix for lip sync issues. See this link for more info:

http://hometheater.about.com/gi/o.htm?zi=1/XJ&zTi=1&sdn=hometheater&cdn=gadgets&tm=64&gps=506_10_1920_910&f=00&tt=8&bt=0&bts=0&zu=http%3A//www.hdmi.org/manufacturer/hdmi_1_4/arc.aspx

It appears HDMI 1.3 had this capability but it was not used until 1.4a came out with ARC.
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Music system
Model 990/7500/Magnepan 1.6 QRs/Technics SL1200 MK2/Aperion S-12 Subwoofer/OWA3/Sony NS75H DVD
APC H15 Power Conditioner

TV System
Large Advent Loudspeakers/ Polk center/Monoprice surrounds/Panasonic Viera 42 inch/Onkyo HT-RC260/Sony BDP S590/Directv


Home Theater System
Onkyo PR-SC886/Outlaw 7125 Klipsch RF-82 L/R,RC-62 center, RB-35 SR/SL, BENQ HT1075, Outlaw LFM1-EX/OPPO BDP-83/Directv
Harmony ONE
Blue Jeans and Monoprice interconnects
APC H15 Power Conditioner