Support for the Radeon HD 3870 X2 may appear in the Catalyst Linux "fglrx" driver in the near future, but chances are it will only be able to take advantage of one of the graphics processors. CrossFire is almost three years old, yet to this day there is zero support for CrossFire/CrossFireX on Linux. Now that we are riding on the new ATI Linux driver code-base there are greater chances that CrossFire support could eventually appear for Linux users. Don't count on it being soon though! Without CrossFire support, you're essentially defeating the purpose of the Radeon HD 3870 X2 as you'll be paying for two GPUs but can only take advantage of one. As a reminder, when NVIDIA's multi-GPU Scalable Link Interface (SLI) was originally introduced to Linux, its performance was poor but eventually improved
The open-source outlook for the Radeon HD 3870 X2 is even more bleak. The fglrx driver would almost certainly adopt CrossFire support prior to the specifications being available in the open, but as the RadeonHD driver isn't performance-oriented, and there are more important areas to address, don't expect it coming soon. Another roadblock for open-source CrossFire support is that the "certified ATI motherboard chipset" requirement could be easily circumvented. CrossFire could technically work on NVIDIA's motherboard chipsets, but AMD doesn't want that, nor running CrossFire on pre-P35 Intel chipsets.
For now, scratch off the Radeon HD 3870 X2 as being a Linux-friendly graphics card or one to put on your shopping list. Meanwhile, later this week we will be delivering the first Linux benchmarks of the mid-range Radeon HD 3650.