That SmartFox thing looks pretty cool. Although something to keep in mind it's just a network layer. You would still have to code all of your games infrastructure on top of it. It talked about using Java, not sure if it integrates with other languages.
The basic layering looks like it would be pretty solid though. Very similar to what we did (we rolled our own from scratch). Looks like it could be pretty solid with the fail over and load balancing.
Not too expensive either at 3500$ for unlimited, although that's a per server cost. So if you wanted unlimited for each of your boxes it could run up pretty fast in cost. Depends on the game though.
Good luck with whatever your choice, it's a long haul coding an MMO, but for a client side Cocos2d works quite well. We have been using it to code our client.