Dropbox keeps your encryption key somewhere. They say that (a) the files stored with them are encrypted and (b) you don't have the decryption key. Conclusion: they have your plain-text key. Thus it is theoretically possible. I take their statement that their employees cannot view your files as "their employees are not allowed to view your files" with some basic precautions surrounding this. However, whoever wrote the code for revealing the plain-text key to the server when you request a file should be smart enough to figure out how to fake being that server.
Only Dropbox can confirm whether it is theoretically possible for Dropbox engineers to peek at user data.