James, if I undersood properly (I haven't had hands-on experience with this stuff ), uuid and evid are two representation (formats) of the same ID.
See
http://community.zenoss.org/message/70515
http://wiki.zenoss.org/ZenDMD_Tip_-_Manipulate_Events
and Jane's paper http://www.skills-1st.co.uk/papers/jane/zenoss4-events/