Flash is always going to be an ideal soultion from pure numbers perspective- it's on 98% of computers connected to the web - however there have been issues with Flash on the Mac - doesn't always play properly. Especially at high bit rates.
Still, Stan is on the right track because most people can see it. Just to be safe, I always prefer one additional mac friendly format - such as MOV or MP4 using the H.264 codec.
However, recently Adobe announced that it's going to be adding H.264 encoding and decoding into flash - which means that flash (using the H.264 codec which looks great and plays well with the mac) will truly be the ideal single solution for reaching you're audience.
By the way, I cover video compression and the best ways to reach your audinece on my new Creative Cow MAster Series DVD - Internet Killed the Video Star - http://training.creativecow.net/
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Aharon Rabinowitz
arabinowitz(AT)yahoo(DOT)com
www.allbetsareoff.com
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