While the good folks at dropbox prepare a lion compatible release for us you can run the current version of dropbox by issuing those terminal commands before running dropbox for the time after installing:
sudo rm -rf /Library/DropboxHelperTools
rm /Applications/Dropbox.app/Contents/Resources/DropboxHelperInstaller.tgz
defaults write /Applications/Dropbox.app/Contents/Info CFBundleVersion 2.0