I have a samsung Galaxy S (Fascinate) that I haven't decided if I want to keep, so I haven't activated it on my Verizon account. I'd like to root it and flash a new ROM (possibly a separate Kernel) but I'm wondering if doing so when it is not connected to any carrier will cause any problems, particularly with the voice/data connection settings?
I doubt it. I've done the same with a GSM Galaxy S (the Vibrant) with no problems. Of course it probably depends on whether you have a really terrible ROM, but rooting alone shouldn't matter. And it might be different for CDMA. But worst case, you can just flash back to stock. My Vibrant picked up the network settings on it's first boot with a SIM, I would again assume your first boot-up after activation would cause the same thing. Plus, if the custom ROM is Captivate-based and not i9000-based, it might have the network settings already built in (and just need to pick up the phone number).
You will not get voice/data if it is not activated on your VZW account. But you can use it as a wifi device, like an iPod. I say root it and flash an ICS ROM to see what the device can do. I think you'll be surprised! Go here for all the info you need to root/flash/return it to stock if you decide to sell it.