Call me a noob, but about a year after TBC I still haven't worked out much use for Envenom.
-It's the same energy cost as eviscerate
-Eviscerate nearly always deals more damage
-Envenom actually REMOVES my DOT poisons, gimping my DOT damage.
-Both take away combo points
So far I've managed to think up 2 possible uses for envenom.
1) High end raids where bosses have a crapload of armor; since envenom deals nature damage I figure it's damage isn't migated by armor and thus might be effective on a few raid bosses (That only counts for muti/subt rogues though, combat ones will be using S&D).
2) PvP versus other rogues. With DOT poison there's no need to keep a rupture on him. You cannot gouge or blind him, unless... exactly, you envenom first to get rid of your own DOT poisons.
Then again, I nearly always go for rupture against rogues and nearly never have the time to switch to DOT poison in PvP. As a matter of fact I have never used this technique or seen it in play. But hey, it's a possible use.
Like Enthalpy said, it might be of some use versus warriors if you happen to have DOT poison up. On the other hand, I've got improved exposed armor in my talent build... If I want a sudden burst (rather than rupture) on a warrior I'll go for a 5pt expose followed by a 5pt eviscerate.