Rancid is a really great tool for backing up and doing diffs on network devices, and is extendable for new devices the developers may not have thought of. After some trouble with Dell Powerconnect switches and my Juniper Netscreen SSG-520 firewalls I came to the solutions described below. I figured by posting this information maybe I can help save other people a little time and trouble. Keep in mind however, these patches come with no warranty and the files they are applied to are the work of the orignal author, I can take no credit for those.
NOTE: Jeremy, who runs rickyninja.net, has updated his Dell extension to work with newer switches. Check out his rancid page first, as most of the information here is now irrelevant.
Rancid does not come with a method of backing up Dell switches by default. Luckily Jeremy S has created an extension for Rancid that adds Dell support and posted it to the rickyninja.net site. The downside to the extension is that, at least on newer firmware images, the extension seems to suffer a problem with paging in the Dell CLI. In the newer firmware images Dell has been wise enough to add a feature to disable paging with the "terminal datadump" command. I warn you I am far from a Perl master, but I have added an ugly hack to make the Dell extensions work again on my switches by adding the datadump command. I have only tested my patch on 3524P, 3448, and 5448 switches with the latest firmware as of the date of this article, so don't freak out if they don't work on other models or older firmware images. Below you find first a link to the rickyninja.net Dell add-on for Rancid, then my patch for the drancid file that fixed it for me™.
Dell extension from rickyninja.net: http://www.rickyninja.net/rancid/*
My patch: drancid.patch
*Note: If you are using FreeBSD you will need to fix the path to the expect interpreter in dlogin.
UPDATE 8/10/09: It appears rickyninja.net is down right now, so I have uploaded my patched versions of the dlogin and drancid files. You will need to fix the path to the expect interpreter if not using FreeBSD. Also note that you need to add the following line to the rancid-fe file for the Dell addition to be seen by Rancid.
elsif ($vendor =~ /^dell$/i) { exec('drancid', $router); }
I don't know what the deal is with Netscreen firewalls and Rancid, but my SSG-520's would not work after a Rancid update a while back. I ended up having to dig up an older copy of the nlogin and nrancid files to make it work again. I don't recal where I found the files, but if you need them the versions that work for me are posted here.
Note that these versions didn't work perfectly by themselves. The nrancid script was missing a curly brace and the nlogin script had all sorts of issues. Below are my patches, but keep in mind they may not work 100% if you are not using FreeBSD and SSG-520 firewalls.