As I am sure you are well aware, there are two tethering apps available from Android Market still, EasyTether and PDAnet. Though yes, I understand that you can tether for free with root, both apps are a one-time-only fee. I paid the measly $18.99 for the "expensive" PDAnet, and am rocking it right now.
Why do I bother tethering? It's actually the ONLY reason I bought the Eris. Where I live, there is no broadband connection. The amount of data I transfer is minimal. But a dialup connection is shoddy at best, so I opted for the whole package.
Back to the point, yes, you can tether with a completely "legit" (no root, official 2.1 OS) Eris. But considering that Verizon, like most providers, frown on any tethering other than their ridiculously overpriced MONTHLY fees, it isn't completely legit.
Just wanted to point that out (again) to anyone who reluctantly thinks about rooting solely for the purpose of tethering. It does come in handy in certain situations.