Do some research on tonymacx86.com. There's several forum discussions about the radon 6xxx series in Lion. Here's one such post where a member mentions how he got his 6870 working right:
http://www.tonymacx86.com/viewtopic.php?f=15&t=64103&p=400568&hilit=6970#p400568
CAUTION: it's a forum meant for hackintosh support, but being that they only allow "vanilla" builds its a reputable site for info. BUT please do a little research and make sure you're comfortable installing "experimental" fixes. FYI I believe the 6xxx support is getting better with some framework included in the new Retina MBP. Basically it's like Apple tossed in support for new gpu's that aren't necessarily being used by Apple configs yet.