Is 650b a better platform than 26" or 29"? I have seen more and more reference to the 650b (27.5") mtb wheel size. Whilst I don't see any need for a new standard, does anyone know of any benefit to have a 27.5" wheel over a 26" or 29"?

While I understand there are currently 650b specific frames and wheels I haven't seen forks yet.

Also can someone please make a 650b tag, I don't have the rep yet. Thanks.
You might want to take a look a this question  which discusses the advantages of 29" over 26".  

650b (or 27.5") falls somewhere in the middle and therefore performance characteristics will be somewhere in the middle of the two. For instance, 650b would be faster than 26" but slower than 29" at same chainring/cog. It would also roll over obstacles easier than a 26" but not as well as a 29".  Also, the weight would be somewhere in the middle.  

It depends on what type of riding you are doing to determine which is the "better" platform.  Also, you should consider how easy it is to find parts in your area for a particular wheel.  Since 650b is only starting to gain popularity, it might be difficult for you to find the tires you want, or to find replacement tubes at your local bike store. Also, I'm pretty sure you have to have a completely different frame for all 3 options, as tires aren't really interchangeable at that large of a difference.  You could possibly put smaller tires on the frame than it is designed for, if you had disc brakes, but it might look odd, and might have weird handling.