I always wondered why nobody ever built something like this and finally I came across this: xat.com has a tool that can simulate real world internet speed. Well, 99$ is not really cheap for something that simply slows down the connection, but the idea is good. For a test you can check out their trial version.

Yes - Flash's built in bandwidth profiler is not that bad, but somehow, whenever I test critical parts in reality I still get ugly surprises. And it doesn't work with loadMovie, which is what I do in 99% of all my stuff.