I'm working on a site with video streaming.. Stats from last year put it at almost a million uniques within the month of july and they expect more this year. this is the first year they'll do the streaming bit.
I would imagine the most concurrent users they can expect is between 150,000 to 200,000. realistically I would think about 50,000 will watch the streaming vid online. it's a race that they want to run live on the site.
I'd like to use a flash/php/mysql solution because it's what i have the most experience working with.. however.. i'm not sure if it can handle that kind of traffic, or is it more of a server issue. if i get the biggest baddest server my hosting company can throw at it, will it matter what solution i go with?
If you get the biggest, baddest server and/or a server farm, then yes, I'd say so.
If it is streaming live then you won't be using MySQL much, use of PHP means little- you just need a good server and/or server farm.
150k-200k concurrent users is a heck of a lot. Presumably once they've reached the streaming video page they won't be moving around a lot, but even so that's a big load on any server regardless of what software stack you're using. And you shouldn't even be THINKING about doing the video streaming yourself.
So, you should seriously consider some sort of CDN -- e.g. Akamai -- to handle the streaming and as a page cache to take the load off your own servers. They have the expertise and the infrastructure to handle this stuff, meaning you don't have to worry about it.
You will definitely need to purchase a streaming host package for the video.
thanks guys.. i knew i would get some good advice from y'all.
i talked to a friend this morning who's used a service called ustream and they handle all the bandwidth, all i have to do is embed the feed into my site.
i love simple solutions. 
Have you checked that the service is able to cope with numbers like that?
Since it's streaming it's going to depend more on your computer and connection to ustream. I've only personally witnessed ustream with about 1K concurrent users viewing a streaming feed. it got a bit choppy at times, but overall it was good.