i was having the same issue but fixed just now,
mine was working fine since it arrived till today morning,
may be it's because i went to my fd home last night and use his wireless network.
the problem of mine is downloading very slow even i just copying a file from the other computer in my network.
first of all i guess that may be the signal problem, so i tried to move close to the router, but it was just the same. and then i reset the pram but it's just the same.
after that i installed xp to find out is that mac os problem but it's the same as well.
finally i found that peoples have the same issue in apple forum,
one of them said changing the channel in the router will work,
and i just try it, it works fine in both macos and xp now.
so this must be airport firmware problem, hope apple will fix it soon,
may be a few months
