I don't see much in the way of Arduino users wanting more processing power. For the tasks that MCUs are used for there just isn't that need. Of course, like cr0sh mentions, there are a number of ARM-based products that will provide that extra power.
If you need high performance then there's only a small price jump to using hardware like a Plug computer or such. Wait until the end of the year and you'll probably be able to buy a Raspberry Pi for ~$25.