PC Engines released information about their new design for an Alix successor.  The new model is called the APU.
I currently use about 30 Alix based firewalls at work.  They are great little devices, they can handle about 30Mbit/s with traffic shaping enabled, and are completely passively cooled and  very low power.  Specifically I use this model from Netgate, the Netgate m1n1wall 2D3/2D13 red.
The new APU platform has a number of nice enhancements that address the current performance walls that the Alix runs up against.

• More memory, 2GB DDR3.
• Faster CPU, 1Ghz.
• SD card support rather than a CF card.
• Gigabit NICS vs the 100Mbit NICS in the Alix
• SATA connector

Possible downsides are:

• Realtek NICS - Realtek seems to have a history of creating buggy hardware that falls over under load.  Some of this can be attributed to the implementation of the NICS, not the Realtek chips.  Hopefully PC Engines will put together a solid implementation using these chips.  Intel NICS would be best but that would probably double the price of the board.
• More power usage.  The spec page mentions a heat spreader attached to the enclosure, which probably means it won't be as cool as the old Alix which required no heatsinks or spreaders.

Release date: Q1 2014.  Test units will be available Q4 2013.
Price: Not announced, but some speculation as to it being in the $125-$150 range, which will be in line with the current Alix.
I look forward to hearing from testers and seeing numbers for  throughput in various scenarios.
Sources:
PCEngines Product Page - http://www.pcengines.ch/apu.htm
PFSense Forum Conversation - http://forum.pfsense.org/index.php?topic=59555.0