P3Droid said on Twitter when someone asked him this: "what if we are running a rom...should we still sbf?" (i.e. go back to stock froyo)
P3Droid: Skip the SBF then
See Twitter Link Here:
Twitter / P3Droid: @brttwrd7 skip the sbf then
Basically he means that if you're on the 2.3.340 firmware, which you should be if you're running Rubix, you can wipe data/cache/factory reset and then install Gingerbread. That's what I did and it worked fine...I did factory reset one more time after I installed it though. Just seemed like a good idea.
Here is how to get back to stock, i.e. 2.2.1/firmware 2.3.340:
Complete Droid X .sbf Flashing Guide That has steps for both Windows and Linux. I found the Linux steps infinitely easier since I <3 Linux and I don't have to mess around trying to get Windows to recognize my phone, which sometimes it just won't do.
Gingerbread is faster than Rubix, in my opinion. Despite Blur being sexier, I still sort of hate it, so when it gets de-blurred by the awesome ROM-ing community, I'll be going that direction. You'll have to decide for yourself

It is fun to mess around with though!
Hope this helps.
Edit: Bald Steve beat me to this, hehe...I am pretty sure though based on P3Droid's advice that you don't have to SBF all the way back to stock and reroot. I think the idea behind telling everyone to SBF is assuming that they installed the unrooted GB leak from last night, in which case you would absolutely have to SBF (which is what I did, and god have I learned 500 new things about my phone in 24 hours because of it) but good luck! Either way, it couldn't hurt to SBF.