The only issue is that DropBox is probably not HIPAA compliant. You can probably make it compliant by taking certain steps.

http://amazingcharts.com/ub/ubbthreads.php/topics/45868/Is_Backup_on_Dropbox_or_Jungle

They also agree that they are not HIPAA compliant:
https://www.dropbox.com/help/238/en