I don't know if you are still interested in this question after so long time, but I'll try to give some clarifications on the subject.

gevent is multitasking network library for python. 

When started in multitasking mode (with the --workers=N option), odoo will lunch several different processes to treat incomming connections. In such a situation some synchronization between these processes is needed. Here comes in hand the gevent library. If gevent is installed odoo will use it's events mechanism to run processes in synchronized way.

More about gevent you can learn here: http://www.gevent.org/intro.html.

In fact, to activate the gevent mechanism you need to use /usr/bin/openerp-gevent as startup script instead of /usr/bin/odoo.py.

Hope this helps.