Is iCloud going to be a threat for dropbox?
It may cause them some lost sales but nothing threatening.
So I think yes, iCloud will have its effects. Not yet though.
Say an iPhone user (who only has a PC) says "Wow, iCloud seems really neat. I wish it worked with my PC though. "
Then his friend says to him "You can try Dropbox. Not exactly the same, but pretty close. Here's an invite. "