There has been some interesting news around about eyefinity being compatible with the Lucid Hydra. This would open up some interesting possibilities.
According to this source
http://www.pcper.com/comments.php?nid=8505
"Windows 7 sees the multi-monitor setup as a single contiguous display from the AMD graphics card, the HYDRA driver can accelerate it regardless of the complexity of the driver and hardware on the AMD side"
If that turned out to be true an eyefinity 6 wouldnt be needed to drive a 5x1 or 3x2 configuration you could do it with a pair of 5850's and have possibly better performance per dollar. 3 x 5850's running with all DVI connections avoiding expensive displayport monitors or adapters could be an option. Using a new series with an older series could also work in DX9 and 10 for 5x1 portrait using a 5850 + 4870.
This source provides some more information about the technology
http://www.pcper.com/article.php?aid=785
"The distribution engine as it is called is responsible for reading the information passed from the game" "There the engine breaks up the various blocks of information into "tasks" that can be passed to any of the 2-4 GPUs in the system"
I'm still trying to find out for sure but it looks as though unlike SLI and Crossfire the RAM contents of the GPU's is not mirrored so if you did use two or three 5850's you would have 2GB/3GB of useable GPU memory and less slow down at high resolutions where frame buffer is an issue.
The major problem with such a setup is the additional cost of the hydra chip on a motherboard but at CeBIT 2010 this was displayed
http://www.tcmagazine.com/comments.php?shownews=33028
An MSI AMD motherboard based on the 870 chipset with hydra! The reduced expense of this particular chipset due to the absence of PCI-Express external switching allows for a hydra chip to be used which can provide a similar role and the price shouldn't be greatly more than a 890FX. This chipset would only allow 2 cards to be used but still should be affordable and a good alternative to a crossfire eyefinity 6 setup for AMD users.
Lots of speculation I know but what do you think?