The algorithm uses requires a large amount of highspeed cache.  Something GPU lack thus GPU will need to pull from global memory (video card memory) doing that make it impossible to operate parallel as the various shaders will be constantly blocking each other's access to global memory.  With a CPU it has sufficient dedicated local cache that each core can operate at full speed without conflict.   You CAN mine with a GPU however you won't get the 100x to 500x performance boost compared to CPU that you see in other cryptocurrencies.

Given the higher cost of GPU combned with higher energy usage it makes mining on a GPU economically prohibitive.

The algorithm doesn't look to see if you are using a GPU, that would be easily defeated.  It simply is based on a "problem" that GPU aren't very good at solving (yet).

Linux only, as far as I know.
Perfomance under Windows isn't good for both Intel and AMD.

I suck at Linux but with some help I pulled it of for my CPU.

I use Xubuntu on a USB stick as a base, first get that up and running.

If you read trough this topic from 4 pages back until now, there's enough information to do it yourself

for those mining on windows, try running 64bit linux in a virtualbox instance...compile w/ appropriate stated flags and Ive noticed much greater kh/s using the linux vm ... then in native windows!

This is because linux is better than windows.  Even when it's virtualized in windows.

would be interesting how good or bad it really is to mine on a GPU. Art, will you release your scrypt gpu miner? has somebody else tested it?

Hmm I'm pretty sure the GPU cost can be lower than the CPU cost (not power/energy, just GPU vs CPU cost)

To buy the first GPU miner costs PC+CPU+GPU
To buy the first CPU miner costs PC+CPU
Firstly, however, the CPU required by the GPU miner is cheaper than that required for CPU mining.
The GPU miner's CPU hardly does any work (if you use Linux and the best ATI driver) e.g. in my case it clocks down to 1200Mhz and only use 4% of a single CPU core when mining.
The CPU miner's CPU needs to be all about performance to get more hashes so will clearly cost more the the GPU miner's CPU thus making the overall cost difference less.

Next, if you wish to increase your hashing power:
For CPU you need to buy another whole PC to add directly to your current hash value.
For GPU you can usually just add another GPU card if you bought an appropriate motherboard with more than one PCI-e slot.

Not a big issue, but just that I've seen a few people say that about costs and it isn't that straight forward at all.

Okay, the bounty entry on the new site is glitchy, so I will just post here. I place a 500 TBX bounty on making and maintaining a collection of windows minerds for various platforms (intel, AMD, 32 and 64 bits, with several optimization tricks).

First person to upload the collection and link here wins (links and filenames must, of course, be descriptive)

Static builds will be preferred (so as to ensure that there won't be a dll hell to deal with )

Gotta run now.

True but currently those GPU can yield far more mining bitcoins.  No chain exists in a vacuum but you are right if somehow bitcoin died and tenebrix reigned supreme you would see GPU mining.  It wouldn't have the multiple that it does on bitcoin but it would be more profitable.  Eventually the cache on GPU will improve and GPU will slowly creep further ahead.  Likely the best GPU aren't the top of the line ones (with top of the line price).  As the scrypt algorithm limits how parallel a GPU can work effectively we may find that lower end (more narrow) GPU offer higher hashrate per $. 

Still as long as bitcoin is profitable there is no real reason to look at using GPU where they are only 1/50th as effective. 

Can't get anything to compile on windows.  Sorry windows users, it's time for linux.