Can I do this with out installing a custom rom or is a custom rom necessary?
I wanted to post partially to answer jbarcus81's question (because I spent most of my day wondering about that) and also to share my specific experience (all good!) with using this method.
First off thanks to EU1 and everyone here for all the fantastic info. I was a root noob, and this way WAY easy for me. It almost felt wrong that it was so dang easy.
Anyhow, like jbarcus, I didn't want to install a custom rom. Really all I wanted to do was be able to wifi tether. Luckily I read through all of the posts here and learned about the engtools.zip - so I didn't need to mess with a custom rom - really had no interest (yet!) - here's how it worked for me:
I pretty much followed the instructions step by step* until I got to the recovery portion.
Once in recovery, I did do the nand backup (probably not a bad idea).
But, I
DID NOT wipe. Totally skipped that step. (I may have opened myself up to some jokes there... whatever)
Then I continued on to the flashing portion, but instead of flashing a custom rom, I just flashed ONLY the engtools.zip (note: once I actually downloaded it, it was called engtools3.zip, but worked just the same).
Rebooted, which didn't take any longer than normal - Rooted, with no custom ROM, just my same ol' eris, but with SU access.
I installed Wireless Tether and am using right now to make this post!
*The ONLY area where I ran into any problems was booting into recovery mode. All of the instructions say to hold Volume-Up and Power, but this did not work for me (and I noticed a lot of frustrated posts on here mentioning the same thing). So, I held Volume-Down and Power, and it worked like a charm. If the instructions ever get revised, you might want to make a note that it may be either Up or Down and if one doesn't work to try the other. Sounds like this was the case for a few people.
Other than that though, too easy. Big thanks to everyone who posted here and all of the developers for helpin' a noob like me enter the world of root. Also, I hope my post maybe helps some folks!
Now, off to see what other doors may have opened for me!