I just watched an old version of "All About Android" on TWiT and this site was mentioned:
Unrevoked Set your Phone Free
The Eris is not shown specifically, but as I dug into their wiki (public:unrevoked3 [RootWiki]
) it mentioned the Eris is supported as the DesireC.
I used this to root my Eris last night and it went rather smoothly.