I typically get around 800-1000kbps via wifi on my iPhone. That doesn't seem like enough for the type of streaming currently used by iTunes and the Airport Express. It streams via apple lossless. 800-1000kbps probably isn't good enough for consistently stable streaming of apple lossless. (Depending on the frequency of signal dropouts and fades.)
On the other hand, I wouldn't be surprised to see some other type of streaming supported at a later date. But I wouldn't count on it either.