Dropbox will not work in osx Lion. It looks like they are doing OS version checking and have disallowed 10.7 Here is a screenshot: http://grab.by/98Ph
There is a forum post about this over on dropbox, if you are currently or will be soon running Lion please jump over there and let them know its important!
And couldn't they just give a warning "Hey, may not work, please don't report bugs" and for reference this is the only software that is not working correctly for me so far.