Is the latest ATI cards like the HD6870 compatible with CS5.5 & the Mercury Engine?

CS5.5 and Mercury Engine will run on any graphic card, the only thing that you will be missing is GPU acceleration on the effects.

Reason I'm asking is because the new ATI cards allows you to use multiple monitors up to 6 I think compared to nVidia's 2 monitors.

That is the essential question, do you want to have support for up to 6 monitors with the Eye-finity capabilities, or do you want to have MPE hardware acceleration? Best would be both, but unfortunately Adobe does not give you that option. More than a year ago they excused themselves by saying that OpenCL was not mature enough, but now, a year later, that looks like a lame excuse.

I expect them to include the support in CS6, because many people are asking for it, though I think CUDA will be more robust than OpenCL.