http://news.yahoo.com/butterfly-labs-announces-next-generation-asic-lineup-054626776.html
Yeah, I linked to that in my posting. This is great news for BFL Single owners. That release says that you will get 100% of your value (in this case $599) on a trade in for ASIC equipment with BFL,
The FAQ confirms this applies to second hand items:
http://www.butterflylabs.com/bitforce-sc-faq/
"Upon release of BitForce SC based Singles & Mini Rigs, all previous generation BitForce products will enjoy a full 100% trade in value when used towards the purchase of the newer generation replacements. This means your current device is protected from depreciation through this evolution in technology. This buy back offer is good for all previous generation BitForce units whether purchased directly from us or from a third party."
You will only get 1:1 though