openbox-3.3_rc2-r2.ebuild should use_enable startup-notification
Hmm, I suspect the startup-notification use flags doesn't do anything useful in any of the ebuilds where is exists, beyond adding a dependency. That's not what Bug 79023 intended. ;)
I am handling openbox now and will get this fixed in the next revision.
Fixed in latest cvs snapshot in tree. Thanks for reporting.