Checking an old email account, I got an ad from YouSendIt offering to allow me to send files to "[my] Dropbox." Intrigued, I clicked, because I rely on Dropbox and like it a lot (despite the fact that it just did a very scary thing when I tried to sync with Eric Goldman for our casebook!). But YouSendIt means its own Dropbox, capitalized. So, much depends on what kind of mark Dropbox is for cloud storage. Is it generic? Descriptive? I bet Dropbox wants it to be suggestive, and in an age of competing internet analogies I wouldn't exclude the possibility that a court would so find, but I'm going to go with descriptive. It's a great name for a cloud storage service that enables sharing with selected other people, but that greatness in communicating its meaning may weaken its conceptual strength as a mark.
Nonetheless, Dropbox has secondary meaning in the field, and I was at least uncertain about what YouSendIt was offering. Descriptive fair use?