[Droid Eris General] lostsync:
I see a lot of people repeatedly asking the same questions regarding 2.1/root/etc, so I made an FAQ for them to be referred to. It is located here:

lostsync.net/erisfaq.html

It's perfectly ok with me if it is reproduced in any way. I'm also happy to make changes and corrections.

Following jcase's advice, the full text is also posted below:

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What is root and why do I want it?

On UNIX-like systems, root is generally the only account that can do anything it wants without restriction. The root account is often disabled or access is not given to the end user for security reasons. Rooting a phone (or anything else, really) refers to obtaining root access through whatever means available. A person would want to root their phone because it allows for capabilities beyond those provided in the original software and gives third-party apps new possibilities (like wireless tethering, for example). It also allows the user to flash custom ROMs, boot images, and otherwise tinker with their phone in ways that it was not designed to let you do. If you are familiar with iPhone/iPod Touch jailbreaking or softmodding a game console, you can consider rooting analogous for all intents and purposes.

Can I root my phone?

Yes, Whether your phone is running 2.1 or 1.5 you can root it using this method

If I have 2.1 leak on my phone, can I root it?

Yes

What versions of 2.1 exist?

There are three 'official' versions of 2.1. These are generally referred to as 2.1 root, 2.1 leak, and 2.1 ota (or new 2.1 leak).

What version of 2.1 do I have?

The easiest way to determine this is to go back to where you got 2.1 and re-read. Barring that, use the following steps:

From the home screen, press the menu key

Select "Settings"

Select "About Phone"

Select "Software Information"

If you see "release-keys" under the "Build Number" heading, you have some version of 2.1leak. If you see "test-keys," you have 2.1root or some derivative of it (i.e. a custom ROM)

Is it possible to downgrade from 2.1 to 1.5?

Only if your phone is already rooted.

Is it possible to upgrade from 1.5 (rooted) to 2.1 and keep root?

Yes.

If I install 2.1leak (or any version of 2.1 currently available), will I be able to receive the OTA update from Verizon when it comes?

Nope

I have installed some version of 2.1. Some apps don't appear in the Market. What gives?

Use this tool made by Jcase

Can I install a custom ROM without root?

No.

I have root. How do I install a custom ROM?

Read Here and Here

Does <some custom ROM> contain <some feature I want>?

Check with the person or people responsible for the custom ROM in question.

Can you explain all this talk about signed ROMs?

It goes something like this: When HTC is ready to release a firmware version, they sign it. They do this by first creating something like an MD5 checksum based on the original contents of the file. That checksum is then encrypted with a key pair belonging to HTC. When the phone tries to flash firmware, it first decrypts the checksum using keys installed on the device. That checksum is compared against the firmware package you are trying to flash. If it matches, firmware installation can proceed. If not, the firmware is assumed to be broken, corrupt, or hacked and is rejected. Nobody outside of HTC knows exactly how they sign their ROMs and the above is really just an educated guess. This makes it difficult, if not impossible, to reverse engineer the signing process as a way of obtaining root. Even if we did know the exact procedure, it would be very difficult to pull off.

I installed 2.1 leak. When will I be able to root my phone?

Yes, Use this method

How did they get root in 2.1 root?

2.1 root was leaked from HTC and was already rooted. Since no procedure was developed by the devs to root it, it's impossible to apply that procedure to other versions of 2.1 that have been leaked from HTC.

What is a 'gold card'?

A gold card is a memory card that has been converted to unlock your device's CID while it is in bootloader mode. It allows you to essentially turn your production phone (meaning one sold in stores) into a development phone (meaning one used internally at HTC for the development of ROMs), which will let you flash any ROM you like. No one has succeeded in creating a gold card for the Droid Eris.