https://wiki.archlinux.org/index.php/Dropbox#Daemon documents possibility to start dropbox as a user. I think it's good idea to have dropbox openrc initscript for Gentoo.
There even are some Gentoo specific init scripts on the Dropbox Wikipage! http://wiki.dropbox.com/TipsAndTricks/TextBasedLinuxInstall/GentooStartup
Added to -1.4.3. Thanks.